Limited "Mailing" Service for a Group

So basically, right now personal messages are disabled on the site for safety. We want one group to be able to message other users, but all other users not in that group should remain unable to pm anyone (we already have it configured so they can contact the mods). So essentially it’s limited personal messaging. The company who manages the forum is usually pretty busy, so we don’t want them to have to enable a plugin or something. And then yeah, the pms should be moderator accessible.