Mods and SE staff.
Moderators who suspect TOS violations will escalate as necessary to Stack Exchange staff and/or the user.
One user can still post representing a group
While maintaining a "group account" that many people access and make use of is a violation of ToS, there's nothing preventing a play-group from having one member with an RPGSE account who asks questions on behalf of the group. For instance: if I'm the only member of my table who has an account and player X has a problem with Y, it's perfectly fine for me to post a question about Y that my fellow player X has trouble with. And in that case I might talk a lot about how "we" have trouble or have tried to deal with X.
So what should you do?
Go ahead and flag things like this if you see them and they raise your hackles. (It's good to know when something's rubbing a "real user" the wrong way, and not just catching our eyes!) A custom mod-flag to say "I've seen this username talking ~as a group~ a lot and am worried it's a group account" is really helpful, since we are only eight eyes and don't see all the patterns that are out there. We'll take it from there.
(But you may not get much of a response. (a) you may not even see the flag as "resolved" for a while--that doesn't mean we haven't seen it or aren't taking action; leaving a flag open is often how we keep something on the front burner for our group-coordination. And (b) if there is a violation we almost certainly shouldn't be sharing details of how it was handled with other users. And by extension (to my thinking) we shouldn't go out of our way to communicate "no violation here!" in some cases and thus confirm by omission violations in other cases.)