When I first started using Trello it seemed relatively easy to add members by their email address. Now it has become a bit more complicated and confusing.
I am trying to add someone (new to the group and to Trello) and I have either sent them multiple invitations, or none. There is now no clear indication that an invitation has been sent out (and is therefore pending).
Initially, persons on the private board were sent an invitation (with the link) to them, and a clear indication that this had happened was the 'INVITE' button, and afterwards, it would show as 'Pending' with the other members shown.
Later on, there seemed to be created a difference between Workplace and actual Boards, so I had to make all members workplace members. This was handy when some (who left the work group) had to be removed. But now, it is really not user-friendly to just add someone using their email address...