Adding a new admin or volunteer coordinator to your instance is easy.  All you need to do is go under Users >> All Users and click New.  There you can fill out the new person's information.  Before you save, you will need to click on the 'Groups' tab and add the user to the 'Admin' group.  This will give them full access to your entire instance.  

If you want to give an existing user's account admin access, just go into their account and click on 'Groups'.  There you can add them to the 'Admin' group.