there is a table called:
in your database where you can set an user manually as an group addmin with a tool like phpmyadmin or MySQL Workbench. The syntax of the table is easy, you need to add the Name of the Group (from the oc_groups table) into the gid field and the Name of the user to the uid field.
Report a bug? / Request a feature? / Developers contact/feedback? See: https://forum.owncloud.org/viewtopic.php?f=17&t=23696