So how do they implement a cross faction guild and yet prevent guild members from grouping together?
I do not see how cross faction guilds could be implemented while still maintaining faction separation.
The same way they're already blocked from grouping with each other or how you can't whisper to somone from guild list?
Simply by removing right-click option. If you see an opposite-faction player, you can check their gear or challenge to duel but you can't select whisper or "Invite to group", these options are simply disabled. Same with guild whispers (which people were asking tobe fixed for a long time lol). You're creating problems with this idea that don't exist