I am sorry for the delay in this. We have received many, many request to make the game "Claimable" as they believe it will only be free for a short time and I'll charge for it down the road.
At this time, however, I have searched and researched and cannot find any option to allow you to "Claim" a game or a game code without doing a one-for-one game key for each user individually.
I am doing my best.
As soon as the game can be "Claimable" we will make another post letting everyone know. With that being said, if you know how to make a game "Claimable" on itch or if you are another developer/ itch admin please comment on this or message me so I can make this happen for you itch users.