What you describe will work, though unless you have a need for individual permissions, it's most easily done on a per-Group basis rather than a per-Member basis.
(And yes, with this method you will be granting them specific permissions to the ACP, not the ACP as a whole.)
Create a new Group, named however you wish, then grant that Group restricted ACP permissions on the ACP -> Members -> Staff, Administrators page.
Everyone in that Group would have the permissions you grant.
You can do it the other way, on an individual member account basis, but that's more administrative work.
Mark H's post in Add Moderators To Support Requests was marked as the answer