Yes.
To do this go to your Group page, then click on "See All" under the members photos. The next page has a dropdown icon on each member to "Make group admin" or "Make group member". Just click on the group member who you wish to appoint as 'group administrator' and click on the dropdown icon and select the option to make them a group administrator.
You can have more than one group administrator.
There must be at least one group administrator, so it isn't possible to use the "Make member" option on a single group administrator.
If by any chance, your (sole) group administrator deletes their World Walking account and your group is left without an administrator, one of your remaining group members should get in touch with us and we will manually designate them as your group administrator in our database.