open BBM > press menu > select add contact > scroll up to "Use Once"
DO NOT CLICK THIS
instead press menu again > select "Request By Pin" > enter Pin number of the person you are trying to add > press trackball > edit message if you wish, if not press OK and you are done.
BB Messenger sort of works like AIM or Yahoo but it communicates by your phone & contacts phone PIN number. You must have their pin number and your contact must have your PIN number. It all starts by sending an invitation to a contact to accept, but again you must have their PIN number first. You must also have a data plan. I personally don't use it, but I'm sure if you use the search forums option you will find more detailed info. Good Luck !