To me, a sentence such as "now we are a committee" conveys more of a sense that finally a group of people has enough members to be considered a committee or has completed necessary requirements for committee status rather than there are no other members. Then again, "bir" can sometimes mean "one" rather than "a" in which case it would convey a meaning of "Now we are one committee," and carry with it a sense that a merge of some sort had taken place -- multiple committees had joined together to become one committee.
As for the phrase "Now we are on a committee," your question is a good one because, in English, it is fairly common to say it that way, but it is a bit idiomatic because we don't literally mean that someone is "on" the committee in a physical sense. We're really saying that someone is a member of the committee. The word "on" in English" can also mean "associated with."
Up until now, the only Turkish word I've learned for "on" is üzerinde or üstünde, but I doubt "Şimdi biz bir komite üzerinde/üstünde," translates into English in quite the same way. I suppose "Şimdi biz bir komite üyeleridir," or "Şimdi bir komite üyelerimiz," would work as well as any other combination of words for "Now we are on a committee."
Please keep in mind that I am not a native speaker of Turkish, but since it has been a week since you posted this and I had a different perspective to share on the English side of it, I thought I'd make the effort to try to answer your question.
Hope it helped!