Depends on your area, depends on how you want to ride.

I'd go to Mapquest or RidewithGPS (my favorite) and plot a route in your neighborhood that won't include much traffic and if possible, only right turns at major intersections. Map a two-k route and ride it twice if you feel good ... three times if you can. Do this every other day for a couple weeks, adding miles if you feel like it.

I'd imagine even a "beginners" group ride would expect that you could use all your controls (braking, shifting, turning) and complete the ride in good shape. And I imagine you would want to be comfortable on a bike before you would want to have to watch out for all those other riders all around.