I don't know where you're staying or when you're going, so you may not actually be participating in FP+...just wanted to disclaim that.
But yes, if you're wanting to be able to use MDE and any other features that ARE available to you, you'll definitely need to get your room's confirmation number and add it to your online account. And I assume they've gotten you some kind of tickets, which is going to be the tricky part if they're not part of a package.
To be eligible to book any FP+ reservations, you have to have a ticket linked to your account. You either accomplish this by having them as part of a package (8-character conf number), or having the separate tickets in your possession so you can link their serial numbers.
I don't think Disney has completely thought through group bookings and how they'll work with MDE/MM+, so if you find anything out, please share your findings here