To edit the group information and the group members of a certain group, first select the specific group and open the group information by clicking on the group name at the top. By clicking on the button "Members" the member overview will be displayed. There you can edit members with the three dots menu. The following options are available: "Make group admin" or "Downgrade to member" and "Remove from group". You can also add new members by clicking the "+ Button" in the upper left corner. These can be found faster with the search function. After that, add them to the group with the "Add" button.
In the group information there is also the possibility to mute the specific group to prevent receiving push notifications. This is only possible for yourself and not for others.