I checked on the web site and there are reservations for 8 available. There aren't as many large tables so that might be why you are having difficulty finding a table for 8. You could also try for tables of 6 and 2 with times as close as possible. When you check in you can ask to be seated together or at least near one another.
It's always better to have a reservation that includes everyone. It's easier to show up with one less person than to show up with an extra person and hope that they will be seated. If you have a reservation for 8, just let the CM know there are 7 of you when you check in. You will not be charged for the "missing" person.