Ok, so if you find that, say, mum's father is already on here under the name Xxxx-1234, you go to mum's profile, click on the edit tab, then down the right hand side select "Add father". This will bring up the form for inputting a new person, but right at the top, there is a little box where you can input Xxxx-1234, the profile number that already exists for him, and then hit enter.
This will link the two profiles and grandad will now be viewable on the family tree tab.
