If you want to remove a person who has already accepted the invitation to the group, you need to be the group owner.
The group owner is the person with a crown on top of his/her profile image.
To remove a person click on the profile image in the top bar and select Remove member from the drop down menu.
Note: You can also remove members who haven't accepted the invitation (pending members). Pending members will appear with a dimmed profile image. You'll see the option Remove Invitation in this case instead.