We are a group from Madison and are wondering which parking lot to park in.

Other questions:

Will there be a parking fee? If so, how much? (Charge/hour)
Is there anywhere else we may park our vehicles? (x4)

Unless you have extra money to burn, park a few blocks away from the Union building (where the con is being held). While most of the streets are 1-2 hour parking only, if you go out 3-4 blocks you can find all day parking, or at the very least longer term parking that the parking checkers don't target as much. Its a calculated risk if you park in a "timed zone", but if you don't mind walking a few blocks there is plenty of free parking around.

I'm not sure on the parking rates for the structures, hopefully somebody else has those answers for you. There is the Union parking structure on Kenwood Ave. and I believe another one a couple blocks away.

Parking info is on the bottom of the location page

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Here is a link to campus parking (a nice map can be found here):

http://www4.uwm.edu/trans/visitors_firsttimer.htm
There is some free parking, some metered parking and a rather large parking lot directly under the union. I believe that parking is free in the union on Sunday, but not Friday and Saturday.

During Friday and Saturday, the cost of parking can be found here:
http://www4.uwm.edu/trans/parking_rates.htm

I think your best option is to park on the street, a little south of campus, or a little north. It can be a ways to walk, but it's free.
If the hotel that you are staying at doesn't have a parking facility, and you park on the street, you can call in an overnight parking permit.