User does not exist. I would like to invite him to a group. It the the problem I met on creating application to improve the cooperation between trainers, players and sport enthusiasts. Allow me to walk you through the problems and their solution.Understand the process Before we go deeper with the technical problems and solution I will describe the process of invitation. There are 3 options of configure join policy:Open - Anyone can joinOpenForRequest - Anyone can send join request. The someone from the ...