this idea is actually the combination of two role plays going on BT at the moment.
I was actually going to suggest opening secret societies to adventurers, but as I started to describe it I realized that if they could simply create a message group - perhaps "purchase" one for a high price to keep it limited - it would serve 90% of my idea.
So - for a high price (100, 200 gold), an adventurer (one time only?) could start a message group labeled "secret society" & invite other adventurers only, in the same manner as the ruler can do now.
This would allow for the formation of anything as innocent as a collective hunting group to as dastardly as a group of highwaymen, but really just make it much easier for a group of characters to communicate without a million clicks, and keep nobles out so they have more freedom to do "bad" things.
I hope this would make it easier to do more collective role playing