1. Most of my touring is solo.
2. In advance:
a. group of two multiple times with my brother, several times with a friend - existing relationships
b. group of two : online postings twice, online contacts once
c. group of three: online posting once
3. Along the way: multiple times, groups from 2-5 typically
4. Organized paid bike trips; TDA - 3 times (~25 across China, ~60 across Africa, ~20 across Patagonia); Spice Roads 3 times (~8 Thailand, ~10 Cambodia, ~4 Vietnam), bikechina - 4.
5. Large rides as events: Ride the Rockies, Bicycle Tour Colorado, Bike Ride Across Tennessee, RAGRAI

So most touring solo, small groups in advance either known or online postings, handful along the way informally, larger groups more in paid tours, very large rides mostly as events.