How to invite a friend to join a board?
Alice goes to board settings:
Alice goes to board settings

Alice invites a member as an administrator:
Alice invites a member as an administrator

Alice invites Bob to join "Art classes":
Alice invites Bob to join Art classes

Bob receives an e-mail to join Alices docboard:
Bob receives an e-mail to join Alices docboard

Bob joins "Art Classes" board:
Bob joins Art Classes board

Bob confirms joining the board:
Bob confirms joining the board

Bob can now work with "Art Classes" board:
Bob can now work with Art Classes board