I second this request.
We've had quite a few customers lately, generally 501c Non-Profit organizations where there may be multiple volunteers or paid employees who may have to periodically maintain or post to their Mailing List(s), ask for this functionality. We've been having to create multiple Mailing Lists, one for each authorized poster, and have them export the Subscriber List from the primary Mailing List and import it into the Mailing List that they are assigned as a moderator of (despite all of them having Admin privileges so as to access the Mailing List menu). Seems far too convoluted of a process for such a simple thing. It would be nice to select multiple moderators from the user drop-down.