I wish to request a mod to be made that sends an automatic PM (which admins can customize) to members once they have been assigned a membergroup.
For example, if I moved Joe Bloggs (a regular user) to the moderator membergroup, I'd like an auto PM sent which I could configure to read something like "Congratulations on becoming a moderator. Here are some rules and guidelines we expect you to adhere to..." rather than sending one manually to each individual member assigned to a membergroup.
Would this be possible?
Sure it's possible. I assume you use SMF 2 from your post history?