You may also want to look on meetup.com, here is what returns for you area.
http://bike.meetup.com/cities/us/wi/platteville/
Also, if none of those are appealing, you could do what I did and start your own ride. May take awhile initially to find people, but you will be surprised how many folks are looking for a group. In the case of my group we have five people, two are slower beginning riders who average 8-11 mph and the rest of us are in the 18-20mph category. How we organize the ride is the slower folks lead the way for the first 5-10 miles (depending on their strength and how far they want to go). The faster riders then split off and put in their miles at their pace and we all wind up meeting again towards the end of the ride. For example, two weeks ago the slow group did 20 miles and the fast 40 of those the first and last five were ridden together. This seems to work pretty well for us, since we are all very encouraging of each other and don't mind taking it easy part of the ride to help others get better/more fit to join us on the longer faster rides.