Most of what's been suggested thus far would fall short of what the OP's looking for, I think - with the exception of Topolobampo, but it will be more expensive. It's highly regarded, though - and maybe a place donor's and the author (whom I'm assuming to be Friedrich Katz) will appreciate. Bayless or his staff, given the knowledge of the nature of the group, might work with you to create something affordable.
Rique's is a place that offers original and good food, but I think the back room is probably a bit too Spartan to be considered by most as comfortable for this event. It's a restaurant deserving of wider exposure, though.
TLO on Diversey is probably a bit far to go, and the environment isn't all that exciting either, IMO - although the food is not bad. I don't see it as a proper environment for this occasion. Now, if you could rent the entire Milwaukee Ave. restaurant space - which I think offers a better ambiance - and the owner creates a special menu for you, that could be both affordable and interesting for your group. 20 people would fit nicely, with enough room to stand for presentations, etc. I don't know if you could get the restaurant to close just for your group - that'll most likely depend upon the day of the week/time of day.
The
Lalo's location at 500 N. LaSalle St. is a nice space, with good private room set-up. The chef there can probably create a special menu for your group, given its size. It's easy to get to, there's valet parking, and street and street-lot parking nearby.
Nuevo Leon's been visited by just about anyone with an interest in Mexican food, so I think a different venue might be more appropriate.
Have you thought of the
Mexican Fine Arts Center Museum as a venue, with food to be catered/brought in? I believe similar events have been held there in the past, and the staff there might work with you on the catering and free or reasonable rental of space. There's an interesting new exhibit for the next several months - featuring the not often talked about presence of Africans/African Slaves in Mexico. The MFACM is highly regarded, and might be a good/impressive spot.