The regroup ability makes use of the Boardgame.GameplayEffects.Teleport sequence. The Teleport sequence has a field, teleportPieceMode, of type TeleportTargetPiece, which specifies what pieces can be teleported (Attacker, Defenders, MyTeam).

I suspect overriding MyTeam to include allies and/or friendlies would result in them getting pulled in via the regroup card.

A related feature could also be:
Overriding Attacker/Defenders to mean not just a single target, but all nearby attackers/defenders. This would make banish an AoE banish, and a mystic's teleport an AoE teleport.

Implemented in #340 - Closing this issue.