"Pending" means the client has been added but hasn't claimed their portal yet — they haven't finished signing up from their invite. To move them along:
Make sure they received the invite (see "My client didn't get their invite email").
Send them your invite link directly so they can sign up — anyone who signs up through it is linked to you. (See "Getting your clients into Homsie.")
Once they create their account, they move out of Pending automatically.