How do I create a challenge for a set number of users?

Open the map you want to create a challenge on - https://www.geoguessr.com/maps/world/play

Click Play and then Challenge.

Tick "Only Invited". 

Click on "Create Challenge" and then "Invite by User ID". 

Copy paste the User-ID of the user you wish to add to be able to play the challenge. 

Click on Invite by User ID if you wish to add more, or remove the ones you have added. 

Start game!

If you want to edit or add more Users you can do so after completing the challenge yourself as well: