I would probably have one person get up extra early and head over to RPR to check in for the whole group (i.e. getting there between 6am and 7am). As others have mentioned, you don't need everyone at the front desk to check-in. You will get a room card for everyone listed on the reservation, though most likely the room won't be ready. No problem as the room key will still work for EPs.
What happens next depends on how early everyone wants to get up and how early you got up. You head back to Endless Summer or you could meet everyone at a CityWalk location of your choosing and then head over to the park of your choosing. As for your bags, that also depends on a few items. What I normally do is see if I can get a late check-out (i.e. 1pm or so.) If the hotel isn't too crowded, they try to be accommodating. If you get it, I'd leave everything in your room and head for the parks for early entry. Then around lunchtime you can head back, finish packing up and move all the luggage to RPR at that time. If you can't get a late check-out time, I'd pack everything up and head down to the concierge desk. They will hold your bags all day for you.
As for getting into your room at RPR, when you check-in, they will most likely ask for you phone # and text you when your room is ready. Your initial set of keys may automatically work in your room, or they may not. I've had to stop by the front desk to get new room keys and I've been able to just go to the room without going back to the front desk.
Use Uber or a taxi to get your bags to RPR. Yes that does have a cost, but it is the easiest path. I've never actually tried to get all my luggage on the buses, then go through the CityWalk security, then take your bags onto a water taxi. It might be possible, but lugging big bags through all that could be problematic, plus depending on what you have in your main bags, it could flag security. So I'd just recommend using Uber or similar to get your bags over. Of course if you have a car, then that isn't an issue. In that case, you could just check-out of your hotel in the morning before heading over CityWalk and put all your luggage into your car. Your car can stay there all day without an additional charge. So you wouldn't be in any hurry at that point. Personally, I don't rent cars much any more. Between the rental charges and the parking charges, I'd just as soon stay in the Universal bubble and spend elsewhere.
