Remove distance from being a factor due to too many elements at play. Ex: Vehicles, group numbers, distance, current activity in game. If [X] Engages [Y] then [X] and [Y] should be engaged irregardless of distance or initial engagement location. Any gang playing together typically are in discord, and even if not its not hard to just say in group channel [X] or [Y] just hands up'd me. It shouldnt be the attacking groups fault if you recruit members who cant communicate.
Theres too many small margins of error otherwise and to fairly enforce you'd need a totality of the situation looked at, and even then it would more than likely be complete admin discretion and opionion. Ex: Gangs where members have autistic 2 letter names or similar names, where you might not know which of [X] you saw but you thought it was [X] Joe when really its [X] Hoe.