I can't answer your question, WYSI, but I can give you a link to a good page:

http://cnx.org/content/m10672/latest/

Unf'ly, your situation seems quite complicated. Clarinets, trumpets and French horns, for example, have different requirements.
On the other hand, many people who play transposing instruments have learned to transpose in their heads as they read conventional music. There is a woman in our choir (plays French horn and trumpet) who can do that.

From her I have learned that guitars like sharp keys and horn players like flat keys. There's not too much you can do about that.

There are a couple of other approaches you might take to a session. If you know the horn players, you could give them the tunes in advance and let them make their own notations and changes. To deal with the problem of flat keys vs sharp, you could have the different groups take turns. When the horns play, the guitarists could sit out, just sing along or do a little percussion.

I believe capos could deal with that problem, too, but I don't think a guitar with a capo on it sounds as nice as one without.