Paul,
I wouldn't worry about waiting until you think you're fit enough/ready.
For the first few rides you will likely find yourself near the back of the group, which is a good place to be.
Riding in a bunch is not just about keeping up.
You will learn the dynamic of that group, the hand/verbal signals they use, become comfortable riding at speed inches from other riders, learn to control your speed without surging (braking to slow down then pedaling hard to catch back up, to then brake to slow down again etc)....plus a whole lot more.
I'm sure if you either contact the group beforehand or turn up early one morning and let them know you're new to bunch riding but are keen to join their ride they will look after you and often a group member will buddy up with you and guide you along for your first ride or two with them.
If the group aren't forthcoming and interested in you joining them, I'd suggest finding another group that is.
Riding with a group is great fun and before long you'll be doing much more than 50km rides and will be giving the same advice to others on the forum

Dave