Bulk Edit Multisites Users Belong to?

I have SCOURED the net looking for an easier way to add lots of users to a new site. I’m starting to become convinced that either:

A) It’s too complicated for a plugin to adjust the database in that way.
or
B) It’s built into core wordpress and i’m completely overlooking the feature.

I have a multisite with many users and many sites. Let’s say I create a new site and all the 500 users of another site of mine need the exact same access to the new site. The only way I know how to do this is to add them one at a time through the wordpress UI.

Sigh…