I wanted to offer up how the Murder in Baldur's Gate campaign describes the Parliament of Peers, in case this is helpful to anyone (especially since bg3 itself doesn't really offer a clear explanation on its own and this wording isn't in the wiki!).
The only role in parliament is called a "peer"—these are usually hereditary positions, mostly held by patriars. (They use this term for them in bg3 itself too, for instance the below:)
"Counsellor" isn't a position that's part of it (I don't personally remember seeing this as a formal title anywhere other than bg3? but my lore knowledge is limited to having played the original games and read the 5e campaigns set in the Gate)—so Florrick and a pre-canon Gortash aren't part of parliament, but have some kind of formalized role to advise them and/or the dukes (or possibly other city institutions). (As "counsellor," not "councillor"—advice-giver, not holder of political office.)
They're a bit long, but here's the section that's an intro to the Parliament, the section on how government functions in general, and more on the Parliament's formation (don't mind the looming Bhaal, he's there for the left-hand column about bhaalspawn):
(There also seems to be fewer peers in the Gate circa 1492 when bg3 is set—Murder in Baldur's Gate is set in 1482 and had about 50. According to the note you can find in Wyrm's Rock, there were only 23 present at the presumably very important meeting to dissolve their political body, which lines up with how many peers Gortash kills right after he's officially Archduke.)
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