From the Group details page, I can manage the group members.
- Select a user then < Remove >, to remove him from the group.
- Select < Leave the group >, to quit it.
- Click on the icon < + > to add members.
- Select the user that you would like to add in your group.
- Validate with < Add >.
If you are willing to add a new person in the group, you need them to be a member of the project first.