How can I nominate other group administrators?

First select the specific group.
To nominate additional group administrators, open the group info by clicking on the group name above.
In the group info you will find the button "Members". Select it to see all members of this group.
You can search for specific members using the search function and select the option "Make group admin" from the three dots menu.
In return, group administrator rights can also be revoked by using the option "Downgrade to member".