Hello LG123!
We've always planned a Dis Meet prior to the actual Sail-away Celebration that takes place on Deck 9 (with viewing on Deck 10). Our groups have found it nice to meet right after the lifeboat drill (4:30) and right before the Sail-away party in the Promenade Lounge.
This area is quite large, is easily accessible from all areas of the ship, and is usually not as busy as the upper decks during this time period. The groups have always had plenty of time to mingle, meet, and chat and then to head up to Decks 9 and 10 for the party.
I'm sure that designating Deck 10 as your meeting spot would work, but your group would need to be more specific as to where on Deck 10 because it is quite a large area. Also, it will be quite busy and crowded at sail-away.
Hope you guys have a great time!