That makes plenty of sense... but it would be hard to imlplement. A "join the 'council if you met the founder or, barring that, a civ already in" (assuming they will be by default glad that someone else is joining the cause) would require some seriosu work... or wouldn't it? (No modder here).
My take was inspired by, at least (meaning: I can't recall what other civ games took the same stance), Alpha Centauri: once the planetary council had its first sitting, everyone had contact with other civs (incidentally: the "what would you give me for X's phone number" as a trade option should have been kept in the "would you like to meet strange, new civs" form, IMO). Ain't it strange that you can observe agreements being made between civs who still have no contact with each other? Are we to assume that, say, the Overcouncil sends pidgeons to some unidentified entity carrying messages like, "Member #3 agrees to mutual defensive pact"? More - and I think it's the definitive argument: when the head honcho is elected, is it possible for civ1 to vote for civ2 in the 'council, even though they have not met him/her, and still not have contact with them?