I am trying to add user to a chatter group from apex. I am getting an error

    "Error [statusCode=INVALID_CROSS_REFERENCE_KEY, code=[xmlrpc=1202, statusCode=INVALID_CROSS_REFERENCE_KEY, exceptionCode=INVALID_CROSS_REFERENCE_KEY, scope=PublicApi, http=400], message=This user can't be added to the group., fields=null]"

Here is the code i am using

 

    lstCGM.add(new CollaborationGroupMember(CollaborationGroupId = lCGA[k].Chatter_Group_ID__c ,
                                                            // CollaborationRole = 'Member',
                                                            MemberId = u[i].Id    ));

    //Debugging
        for(CollaborationGroupMember cgmr : lstCGM)
        {
            system.debug('Processing the CGM record ' + cgmr);
        }
        
        if (lstCGM.size() > 0)
           {
               // Insert the CGM Records
            Database.SaveResult[] lsr = Database.insert(lstCGM, false);
                
            // Loop through the save results writing errors to the debug log
            for (Database.SaveResult sr : lsr)
              {
                if(!sr.IsSuccess())
                {
                     Database.Error err = sr.getErrors()[0];
                      System.debug(err.getMessage());
                  }
             }  // END LOOP through the save results writing errors to the debug log
         }// END IF there are records to insert, insert them.

The debug logs return the correct ids

 

  Processing the CGM record CollaborationGroupMember:{CollaborationGroupId=0F9E00000008cpwKAA, MemberId=005c0000000R2kQAAS}
    USER_DEBUG|[275]|DEBUG|This user can't be added to the group.

Checked the ids on the browser and they were valid.
Any idea what should be causing this?