It is not possible to check whether an invitation has “bounced” on the Citizen OS platform.
In the case of a group invitation having been sent to an email address that does not actually exist, the invited member will still be included in the group’s member list as “Pending”. This is because the system cannot recognise that the email invitation has bounced.
The only way to find out whether an email address you invited may have bounced is to send an email to that address yourself, and see whether you get a “mailbox error” automatic reply.