1. If you started a group you control the admin features of that group.
2. Click the group you want to administrate. On the right side of the screen you will see the list of participants. To adjust their settings choose which participant you wish to change by clicking the three dots next to their name.
3. You can now remove that participant from your group by clicking remove.
4. You can also promote them to moderator.
5. A moderator has the same admin rights as you, to remove, add and make moderators of group members.
6. You can remove them from this role by performing the same procedure.