Usually, as far as I know, a collab on Tapastic is a trade of comics, meaning a guest comic exchange. To have two or more people work on the same comic through tapastic is a bit rare, but not impossible.
If you are looking for a collab, first you have to get in contact with the creator. If they agree, they will tell you how to contact them or how to send the files.
Usually it'll be an email or some other form of messaging system.
This is all pretty basic stuff to think about, so I wouldn't worry too much on the logistics, rather with whom you'll be collaborating with.