Four headsets can be paired together and can talk 1-to-1, or you and two friends can be in a 3-way conference, so long as you are within range of your two friends. In a three-way you are the 'hub' and your two friends are the 'spokes'. OK, so far?
A four-way conference is a bit different. Let's say you are A, and your friends are B and C. A connects to B, B connects to C. For the 4th person 'D' to join in, they will connect to (and must be previously paired to) C. So, the signal is repeated from A to B to C to D.
Now, let's take rider 'E' for example. Rider E can be previously paired to C. If rider D goes out of range, and rider E comes within range of rider C, then rider E can join in the conference. But the 5th person is the odd man out.
The Scala Rider G9 can also do a 4 way conference, but can pair 9 total headsets for 1-to-1 communication, Sena can do 4 total for 1-to-1. The Interphone F5 can have a 6-way conference, in the A-B-C-D-E-F configuration, like the Sena's repeater-mode 4-way. We have the G9's and F5's as well as the Sena's.
For Sena SMH10, I look at it this way, two bikes, each with rider and passenger, this all works great. Otherwise it can be a bit confusing. The best way, IMO is to use the SR10 2-way radio hub along with the headset, and FRS/GMRS radios. This way you can have as many people was you want all on the same radio frequency, and everyone can be a part of the action. Plus, the SR10 works with non-Sena headsets, so that eliminates the issues of everyone having to have the same brand of headsets.
Now, the big question is **Am I making sense, here?**