Only the group owner can remove a group member from a group.
Go to the Groups list by selecting the Groups view in the top bar (make sure you have a specific account selected in the sidebar accounts menu if you have more than one account). In the group management page find the group member you need to remove and click the Trash icon on the right side of that group member line.
The group member will be removed from the group and lose access to any of the information (spaces, chats, files) that was available to them through this group.
The removed group member can be added back to the group at any time.