I've always split the list in two when we get over 20 staterooms. You can then deliver to just your list or to everyone, but at LEAST to your list.
I always feel a little bad about the folks that want to join in after the cutoff, but I guess they can do their own thing if they want. I've only seen two people want to join after the cutoff though - and that was on 3 different cruises, so it shouldn't affect too many. We can also shorten the cutoff to maybe 6 weeks or so if everyone agrees.
How about a 'latecomers' list that we can deliver to if we're able? Like you (Jen) said, they can have their own group, and we can deliver if we'd like. Some of my gifts are easy to make, and I can always pop a few more out.
Thoughts?