What's a group ride? Nah seriously then I don't train for anything as I've never raced but we have moved pretty often in the past and kids came along and now I live in a mining town with no LBS & just motorbike trails.
I've always ridden solo and exploring the surrounding bush and trails by myself has always been one of the best parts of riding, no time to get bored of doing the same circuit all the time that's for sure.
At one time my husband did come on a ride or two, but he's not into it and doesn't like going up hills so in reality I did prefer to go by myself, like others have said those sort of rides are treated more as group rides. But from that I came to understand how you would feel like you're not very good if other people are faster / better than you and I can see how you end up pushing yourself more when riding with someone else.
Of course the down side is not really knowing how you would go in a race or where you fit in a group, but hey maybe one day.
It's funny at times I've entertained the idea of putting a notice up at the local shops just to see if there is any mountain bikers in this town, I know I haven't seen anyone out on the trails - well one bike once in the 9 or so months we've been here.
My group rides are out the back on the dirt trails and rolling over the jumps with my 2 & 4yr olds on their "firstbike" type bikes.
I guess in an ideal world you would do a bit of both: some social rides with a group, some solo rides to maintain your riding essence, training rides with a buddy or two who have similar abilities to yourself.