On 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 >.
You can create a group only with member of the project. If you wish to add a new person outside of the project, first, add this person in the project, and then add to a group.