Short description elten's answer do not explain a lot.
Elten and OS 3.0.5 rc3
Steps to reproduce try to invite someone who is in a group or wait for accept. Elten say's error. It should say exactly the reason eg. this group has been already invited, or is in your group.
dawidpieper merged a pull request by td-programs, #175: Handle duplicate group invitations in commit c098b4a Handle duplicate group invitations (#175).
This release is at least 7 days old, there are fewer than three confirmations, no confirmation from the thread author and no negative answers, and this thread has had no discussion for at least 7 days. This thread will be archived in 7 days unless a new discussion starts or someone answers No. Three confirmations will start a new 3-day notice period; confirmation by the thread author will start a 1-day notice period instead.