@harishchouhan, 
Thanks for your post. Unfortunately, it is not possible to obtain a URL to send to friends to allow them to join a group challenge without being invited directly. Additionally, it is not currently possible to send invites to athletes who do not follow you on Strava.Instead, it is necessary to send an invite to the athletes you wish to join the challenge.
 
To invite athletes to a group challenge, open the Strava mobile app and navigate to the Groups tab > the Active sub-tab > and then tap on the group challenge you created. From the group challenge's screen you will see an orange 'Invite Athletes' button displayed at the top.
 
I hope this information points you in the right direction. 

Thank you for your reply. This would make it difficult since I can't follow more than a thousand athletes. Is it possible for people who I invite, to invite their friends?

Also, will I be able to see everyone's ride in case I need to flag someone for cheating? 

Is this solved yet? 

Is it possible to join a challenge as a community?