Let’s see how it works:
I invite firstname.lastname@example.org to a new mural for the first time.
Megan accepts the invitation and enters the mural.
Megan is now part of my address book and I am part of Megan’s address book.
Next time I want to invite her to a mural, she’ll appear as a suggested user.
This way, you won’t need to search for any complex username, they’ll just appear on the suggested list.