Grand gatherings question

[Disneyfanfour]

Here's a question that probably has already been answered somewhere:

Can you include other people/friends in your grand gatherings if they're not registered with you. For instance, we have friends coming up to WDW for a few days and we'd like to include them in our grand gathering festivities, but they're not signed in on our group. Can I include them when making my reservations?

Thanks for any help here.