My grandsons 16 friends will be purchasing one day tix separately. Can they link them to the MDE account my grandson created for this trip as they purchase them? Or do all the tix need purchased at the same time? Thanks.
Any who have their own MDE would link their tickets to themselves via their own account. If they're linked to your grandson via F&F, your grandson's MDE account will recognize they have a ticket (when trying for RotR BG, for instance). Any who don't have their own MDE need to give the ticket number to your grandson, and he can link the ticket to that person.
Each person needs to have a profile in My Disney Experience, to link the ticket to. That can be their own account, if they want one, or as a profile within your son's account.