Cheap, Delicious, Fun Mexican Group Dining

[sf2167]

Chowhounders, you have been so good to me from recommending places for my birthday or a nice restaurant to go to with my parents, you have never failed me. So here's the next question:

My best friend would like to have a birthday dinner on Sunday, April 17th, and wants the following:

Delicious, as-authentic-as-possible, Mexican food
12-15 people
Fun atmosphere
Not over $20 an entree
Good chips & guac
Any downtown neighborhood

Looking forward to your responses so we can make a reservation (time flies fast) soon!

Thanks!

[sf2167]

I should add that South Brooklyn is also an option, but I know I'm on the Manhattan board...