I don’t think that there is an elegant solution for your problem.
There are lots of reasons that a creating a group might be too heavy-handed, but you can make groups visible only to members of the group.
I don’t think that there is an easy way to bulk copy-paste recipients into a PM, but sometimes things “just work”.
You can change the title of the PM, but it’ll still be littered with the previous discussion (unless you delete those posts?).