I think groups are actually called servers. If that's the case, a server can be created to run on its own (like any old server), and anyone can join it who has an access link (which the creator can give out).
Yes that, setting up a decent server securing it from "griefing" first. A also figured out the issue with old packs, 1.13 introduced a overhaul to resource pack formatting, so I am an hunting for some sort of automated tool to covert a old pack to new format for painterly fans to use before they apply the continuation patch.
Like any typical server that doesn't have proper safety protocols applied, a few examples: it could be flooded/overwhelmed if not secured, have people enter and make unwanted changes if proper permissions aren't set, or, most common I've seen, someone could abuse text chat (spam) or voice chat (constant interference).