Sounds like fun!!! Usually open mics are just that - a microphone. Those are more of a singer/songwriter thing. This sounds like they are setting up for pickup bands.

Do they have a "house band" that you could just sit in with an play guitar?

If they are providing amps, you would just show up with your guitar, and maybe some pedals. Your first time out I would skip the pedals. You don't want to turn it into a big deal getting setup.

Let us know how it goes...that sounds awesome!!