click the drop down, by the game icon image and allow whatever part of the group the player is in(admin, member, etc) to edit the game, if you want only him, he needs to be in a group all by himself, and I am pretty sure that he needs to be part of the group…I think…
You aren’t able to grant individual users edit or management permissions to a group game. I’m not sure if there’s a real blocker to doing this or if Roblox just decided to solve a nonexistent problem again but you may only grant play access to individual users. If you need a user to be able to edit your game, then you need to create a separate role to grant edit access to.