I've done a few of these with my homebrew clubs. Depending on what your club, and the brewers involved are like, it can be anything between a smooth process and herding cats.
First tips that come to mind:
Recipe:
Have everyone commit to a recipe, and have them swear to it, twice. Then expect that it won't go as planned anyway. You'll want everyone to adjust the recipe to their own efficiency, but you'll want everyone to be pretty close for best results. Of course, you're blending them all together, so some minor differences will average out, but if everyone makes their own changes, you'll end up with recipes that have three different base malts, six different hop substitutions, and four different yeasts "because that's what i have on hand". Also try to get everyone on the same page with the time that the beer will barrel age. You'll find that some brewers are antsy to get it out of the barrel after a month, and others are fine to leave it there for a few years.
Timing:
Give everyone plenty of time to brew and get it transferred to a keg or some other transportable container. You don't want someone brewing at the last minute and transferring still-fermenting wort to the barrel. Better to have someone's beer sitting in a fermenter or secondary for a couple weeks to give everyone time to brew and ferment.
Logistics:
Plan the barrel fill date well in advance. Make sure everyone knows about it, and can either be there to help transfer into the barrel or have their beer there ahead of time. Also be sure you have a good place for the barrel to stay while it ages. I've had beer in barrels in friends' basements and at a LHBS that has room for barrel aging. It's not easy to move a full barrel, so be sure that it's where you want it to be. If possible, have it on a heavy duty furniture dolly so it can be rolled around a little, but keep in mind that a full 50G barrel is over 500lb.