I've often thought that the best syndicate would be two gliders between four partners, or maybe three between six. It's rare to see more than half the club members, even on a good day. Intuitively, that kind of syndicate would seem to almost guarantee availability except on the most glorious of days, obvious some time in advance.
Retrieves have never been a problem - glider pilots seem to quite like helping each other out, and the promise of beer is always effective. Lack of a pre-agreed crew is more a psychological barrier than anything else; since I made rigging gear which allows me to rig and derig solo, I've realised that my fall-back option is to hitch/taxi/train back and then fetch the aircraft myself. I've never had to do this, but the knowledge that I could has let me set off XC much more often than previously. I have a dream 500k planned - Rougham (Suffolk)-Sarnesfield (Herefordshire)-Rougham, most of the width of England out and return. If the right day turns up I'm going for it, crew or not, and will write off the following day in advance.