Making custom user roles require admin validation to register

I’ve created a few custom user roles and modified functions.php to enable new users to choose from them during registration, but I need all but one of those roles to require admin approval before the role is granted to the user.

How might I go about that? My best googling only turns up advice for requiring all user registrations to require admin approval, when I only need registrations of certain user roles to be moderated.

Hope somebody can point me in the right direction!