Worth adding for infantrymen willing to sign-up, if you don't have an active cavalry regiment, the roster rule doesn't apply to your team. It's only a limitation for the members of the cavalry community.
So let's say Rikkert wants to create a merry band of cretins from the 15thYR and have them partake in that event, they can all partake under the same team, so long as all of them are not active in the same cavalry regiment at the same time.