Search search search first. Many many many times this has been asked and answered ( I was a little guilty myself)

Oh alright, here's the best single source of info on RAF on this site to my knowledge:

New Recruit a Friend Features

But to answer your question, if X is linked to ABCD putting a 70 in the group will douche your xp and you won't get bonuses, unless you drop or switch out the 70 at quest turnin time.

You can't boost with a 70 AND get the RAF link bonus, it's one or the other. I had my best friend invite me so I can boost with my 70 and not waste the free levels completely.

If you link X to ABCD and create a lowbie on X to group with the new guys, all the bonuses apply.