Hey Ahsan,

Unfortunately there is no way to add players to a challenge once its been created.

The only thing i can think of that might be a solution would be to get the players currently in the challenge. You can see the API for that here.

Then you could create a new challenge with those players plus the new players you want to add, and then automatically have each player accept the new challenge.
However, this wouldnt preserve any info you have about turns in the challenge, so it might not be suitable for your example.

Sorry we couldnt be of more help.
Sean