If you do not have a partner lined up then, when you register for a BBO tournament, click on the Partnership Desk tab. If there are players already listed there, you can invite one to partner you. If not, or you are wanting to offer to be a fill-in only if needed you can add yourself to the list and wait to see if someone else will come along and invite you to play.
You can even require any interested partner to pay your table money which might be quite appropriate if you are there only to offer to be a fill-in if needed.
If no-one takes up your offer before the tournament starts, you can BBO chat to the director just before or immediately the tournament starts in case he needs to fill in a half table - the director's BBO username is listed in the Tournament's Details tab.
