If you created a subgroup in your Yammer network – you may want to make someone (other than yourself) the admin of that group. The process is really simple.
1) Navigate to the group that you wish to modify – and choose the Settings gear in the upper right hand corner.
2) In the Member Management Section – choose Add and Remove Admins from the Manage Admins section.
3) In the window appears – you will see all of your members – click the drop down arrow beside the user’s name and choose what you would like to do with that member. (In this case – Make Admin. If they are already an admin – you can revoke that admin access). I have also included a screenshot of how to revoke admin access.