If these are competitive rides, figure you’ll go about 30% faster than your solo pace, provided you are fit, and your solo rides are done at a good pace, and that you hang with the guys at the front of the group. Group rides start slow, pick up speed gradually become fast, and usually break up later on when the faster riders pull away. This is the usual formula where I have ridden in America and Europe, I can’t imagine it being much different in Australia, except being on the wrong side of the road.