What permissions are required to test as another user?

I believe if users are allowed to share the app, then they can test as Role.  There is a checkbox on Role setup

"Manage users and share the app

Thanks for the reply!

I do see how they're able to test as another role. I'm interested in how they can access the 'test as another user' feature that was rolled out last month. This only seems to be available to those user roles who have the "edit app structure and permissions" box checked. 

Don't forget, they just rolled that new feature back somewhat, because you can't modify or add or delete any records when testing as another user. They can only access the app as read-only.