I feel bad for your, DP, I really do. So sorry if this sounds heartless but he's a grown man - he needs to sort this out himself.
You'll both look absolutely foolish if you approach the wives group about this issue - you're not his mum seeing the school about some name-calling.
He needs to either:
A; remove himself from the 'main' WhatsApp group, block the men's numbers and move on with his life without these arseholes
B; straight out talk to the administrator - either ask to be added (cringe and plays into the bullies hands) or tell the administrator he's a nasty cunt and then see Point A
C; carry on as usual but make it clear that he's fucked off about not being added, and massage things towards coming to a head/resolution. For example, when DP is invited to things with shorter notice, don't go, and say "Well I would've come but I haven't had enough notice because it was all arranged in your secret little WhatsApp group that you've excluded me from"
TBH, I vote Point A - life's short, surround yourself with people who enrich you, ditch everyone else.
But you absolutely shouldn't go wading into this. Just listen to DP, be sympathetic, be supportive, but don't bring it up with the wives under any circumstances.