Well, the new member needs to have gotten in somehow, and if they kept the note that Ronan wrote for them it should work. (Especially since he rarely if ever leaves the manor and would only have been able to get newbies in via that method.)

Nothing saying we can't bend the rules of the Fidelius too, though! Who knows? Maybe the charm works differently in the AU, and that's how the Potters all died? ^^

EDIT because I got ninja'd in my own thread :C
- Well, we could do that then! We could even make a letter work if you wanted - the other wards would prevent unwanted magic like tracking charms or the Trace from following it inside.