I'm sure you've figured out this requirement, but syncing is two fold, which could be elegantly blended into one. There is syncing between my devices (iOS and Mac), and then syncing between my band's devices (perhaps pooling/syncing).

Take a look at BT Sync, they got this working really well. Not the world's finest UI/UX, but rock solid and very fast. Most importantly, no need for a server. Maybe they have an API or license their engine or something.

In any case, it's the feature that's begging to be implemented.