As far as I remember If there is only one site Manager ,the site Manager cann't leave the site. So the site Manager's account is still available and can be used to change a member’s role
Now Only a Site Manager can manage site membership by changing a member’s role
You mentioned that you created a site and then invited one user as manager and other 10 users as consumer. Is that correct? In that case, as you have created the site hence you become site manager for that site. Any site in alfresco share should have at least one manager. Hence, if there is only one manager then it will not allow him to leave the site. In your scenario, you being a site manager (as you created a site) can invite other member as manager if you want.