how do i actually bring them up to speed XP wise?
I'm not sure I really understand the question, so I'll just brain-dump!

Only the lower-level character(s) of a linked team get the RaF XP bonus, which means the lowbie catches up anyway. You can also grant a level to the VETERAN (i.e. "inviter" (sic)) side of a linked account (right-click on their party bar, there's a new option), although I haven't given that a shot yet.