You can't add Genie Plus until the day you want to use it. The only guests who can add it in advance of the specific day are persons with a resort reservation plus tickets, who can add in advance only if they wish to purchase it for every ticketed day in their stay.
If your group likes to sleep in and then go to the pool before hitting the parks, not sure why you would want to buy
Genie+. You could probably use ILLS if you are able to grab a later time for them.
I have a friend who is at HS now with her sister. She wanted to use it while her sister didn't because the sister prefers the shows to the rides. she says it is probably not worthwhile to have one member of a party use Genie+ while the others do not, they would have to split up a lot.