On any profile in question, towards the bottom of the page on the right side column, in the Collaboration section, there's a link called "request to join the Trusted List"
The manager will then be notified by email, and there will be a link included in that email to approve.
Otherwise, on the "My WikiTree" menu, click on "requests" menu item, and it will show any outstanding Trusted List requests waiting for approval.
I do not believe any Trusted List member can remove you as manager. Only a co-manager can do that. (I could be mistaken on this detail).
You can promote Trusted List members to be co-managers, but I'd suggest not doing that unless you know the person very well.