Address Book > Groups: not accepting additions

I have a particular Group I have been trying to add additional contacts to.

I don't need them to be in the main (individual) listings of Address Book, so I have been trying to include them via the Group > All Properties > Name & Email > Add - but the Group will not update. It has 42 members in the Group and that is as far as I can go.

I can update addresses already there (and most of these were added originally in the same way), but I cannot add any more. Is there some sort of weird limitation to the number of names in a Group and, if so, any way of getting around it? Does Groups only accept a certain number of additions not included in the main Address Book?

I know Address Book has a problem with Names over 150 characters, but this in not what is happening here. Any thoughts?

(XP Home, SP2)