What Sade said: in that example you would need at least a 3 to add a henchman. Once added, the group would have a Group Experience of 12, as it would include 4 henchmen, each with 3 experience. Otherwise it would become impossible to add to some more experienced henchman groups (eg a group of 3 henchmen each with 5xp require an xp roll of 5+ to add a new member; if it were a roll of group xp, you would need an unobtainable 15!).
So just the basic value on the Experience Track, not the Group value.