In case you want to add a new member to an existing public or private group, the admin of the group will have to follow the following steps
The same process will have to be followed to add a new member to a public group, alternatively, you can ask the member you want to add to the public group to join the public group themselves by clicking on the name of the public group and clicking on OK in the next dialog box that opens.In order to remove a member from a group, follow the following steps.