Dm Guide: Running The Game: Group Size
DNDs rules and published adventures generally assume 4 to 6 players plus the DM.
Small Groups: A group that contains fewer than 4 players might find combat encounters difficult especially if the party lacks important capabilities. You can compensate by reducing the number of monsters in a combat encounter or by giving the party resources they need such as Potions of Healing. You can also add party members.
Dm Controlled Adventurer: You can make an adventurer character of your own to accompany the party. This is a rewarding way for you to roleplay with your world but keep in mind that you'll have to run this NPC in combat. Be sure to keep the player's characters in the spotlight and don't take away the players agency by having your character make the decisions for the group.
NPC Party Members: You can add nonplayer characters to the adventuring party. Use Stat Blocks like those in the Monster Manual to represent these supporting characters. If you don't wnat to run these NPCs yourself invite one or more of your players to take on an NPC as a secondary character. These NPCs might be apprentices or employees of the adventurers so it's natural for the main characters to take the lead in exploration and social interactions while the NPCs fade into the background.
Players with Multiple Characters: One or more of your players can each play 2 characters. Running 2 characters at once is a challenge so make sure those players are comfortable taking on multiple characters. This approach works best in a game that's focused on combat capable characters. It can be difficult for a player to roleplay 2 characters at once. You might suggest that the player focus on roleplaying one character while relegating the other character to a supporting role.
Large Groups: The biggest considerations with large groups are maintaining order at the table and keeping combat from becoming too slow.
Party Leader: Consider having the players designate a party leader who is then the only person who tells you what the group is doing. It becomes the leader's role to work with the rest of the players to find consensus on what the group will do.
Speeding Combat: Players who have to wait a long time between their characters turns in combat are susceptible to distraction. Consider these tips to speed combat with a large group:
Be generous with information: If you tell the players what the AC of their opponent is you reduce the steps of interaction needed to resolve an attack. Instead of telling you a number and asking if it hits a player can simply tell you that an attack hits and how much damage it deals perhaps adding some narration for good measure. In the same way if you know each character's AC you don't need to ask whether a monster's attack hits.
Help Players Keep Up: If a player isn't sure what to do on their turn in combat help the player decide by offering a quick recap of the state of the battle. How many foes are still standing and how hurt do they look? What's the most immediate threat to that character?
Make Initiative Obvious: Display the Initiative order to your players so they each know when their character's turn is coming up and can think ahead about what their character will do on their next turn. Using Initiative scores and perhaps seating the players in Initiative order can be helpful with a large group.
Roll Handfuls of Dice: Encourage players to roll the dice for their attack rolls and their damage at the same time. You can do the same.
Structured Turns: If you find yourself in a situation where individual players are having trouble getting a chance to do things during exploration or social interaction have the characters roll Initiative and act in Initiative order just as you do in combat. Taking turns ensures that everyone has the chance to do something. Use this approach sparingly as it can feel artificial and sometimes slows down the game.
Abesent Players: When one of your players is absent what do you do with that Players charcter?
Fading into the Background: Have the character simply fade into the background. This requires everyone to step out of the game world a bit and suspend disbelief but it might be the easiest solution. Act as if the character were absent but don't try to come up with any in game explanation. Monsters don't attack the character who returns the favor. On returning the player resumes playing as if the absence never happened.
Narrative Contrivance: Decide the character is elsewhere while the rest of the party continues the adventure. Come up with in game reasons for the character to temporarily leave the party and rejoin later such as following up on a rumor or reporting back to the party's patron.
Substitute Player: With the absent player's consent have another player run the missing player's character or run the character yourself if you feel you can do so. Whoever runs the character will need a copy of that character's character sheet and should strive to keep the character sheet and should strive to keep the character alive and use that character's resources wisely.
Give absent characters the same XP that the other characters earned each session keeping the group at the same level. Some groups like to work out a policy regarding how many missing players is too many to proceed. For example your group might play as long as no more than one person is absent. If 2 or more people can't attend a session consider playing a short adventure with different characters and perhaps a different Dungeon Master or bring out a favorite board game.
Incorporating New Players: When introducing a new player to the group revisit the group's expectations and limits. Then have the new player create a character who is the same level as the other characters in the adventuring party. If the new player has never playerd DND before and the rest of the group is higher than level 4 consider taking a short break from the campaign and having everyone play a new level 1 character for a session or 2 while the new player learns the ropes. If you're incorporating a new character into the group in the middle of an adventure work with that character's player to come up with a story hook for how their character joins the group and make sure the player is happy with the choice. Here are some examples:
Long Lost Friend: The new character is a friend or relative of one of the adventurers. Alternatively the new character is connected to the adventurers' patron or a member of an organization the other characters are linked to. In either case the new character has been searching for the group perhaps bearing important news.
Rescued Prisoner: The new character is a prisoner of the foes the other characters are fighting. When rescued this character joins their group.
Sole Survivor: The new character is the sole survivor of an ill fated group of adventurers. The new character might be able to offer a clue to help the party avoid the same grim fate that befell the other group.
Special Guest Stars: The story hooks for incorporating new players can also work for occasions when you want to bring a player into the group for a single session. For example you might have a friend visiting from out of town who wants to join your game briefly. Or perhaps you have a player you're thinking about adding to the group but you want to make sure they'll be a good addition. Incorporating an occasional guest player is also a great way to maintain a roster of players as backup in case one of your regular players has to drop out of the game.