Logically guessing, I think the respawn type of "SIDE" is supposed to let you respawn into any friendly unit, which therefore can be in another group. However, it doesn't seem to work properly.
I have a mission where I tried this type of respawn. I had the player/host (server), and 1 client player. The client player was a member of an infantry squad. When he died, he respawned as the gunner of an M113 that was *not* part of his original group. However, even though he had respawned, he was unable to fire the machine gun, nor could he disembark from the M113. So, it was obviously quite buggy.
Therefore, for all effective purposes, there is no way to respawn into another group.