User is already a Member of this group.

When we deactivate the user who is member of Chatter group, the user will be invisible from the Chatter Group.We not able to find the user using this query :

SELECT Id, CollaborationGroupId, MemberId FROM CollaborationGroupMember WHERE CollaborationGroupId =: groupId

Also the user is not inside Chatter Group --Members .

But when we activate the user, automatically it will be appear in Chatter Group.

All of this is handled by Salesforce.

Please note this when we plan to add user into the Group automatically because we may bump into exception below : 

User is already a Member of this group. 

So we need to verify if the user is a member of the group before adding it.