Brian,
I think you can do something like what you want for subsites by making the moderating user (MU) an admin of a subsite, then the MU can manage users at subsite/members
For example, example.com/subsite1/members
This is not exactly the same however, as users will need to register to the main site first for this to work, and permissions can be applied to other subsites on the website, just not the main subsite.
For example, you can give the group members at example.com/subsite1/members specific permissions for example.com/subsite2 etc. That way any member of subsite1 (determined by MU) will have the pre-arranged permissions for subsite2.
Note that if a subsite/members admin adds a new user directly through the UI instead of just adding a registered user to subsite1 membership, the notifications such as invitation are buggy (may not work for the new user).
Hope this is clear, if not I'll be glad to explain in more detail.
cheers,
Torben
