I would say yes. Each room has a set number they allow in and from there they will let 2 more people in at the time of the event. If you have. Less than a full room you may get lumped in with other people. Reservations are definitely a good thing.
It's a good idea, especially if you have a sizeable group.
You should make a reservation if you want to get in a certain room at a certain time. These things usually book up ahead of time so better safe than sorry.
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