This feature is currently in Early Access.
What you'll need
If you have "Platform management: Sites" permission, you can manage the membership for all sites in your organization.
Add a site manager
Click your organization name on the lower-left corner of the page and select Sites.
Click the site or level name.
Select the tab Managers and then click Add managers on the right-hand side.

In the side panel, click the dropdown menu and select the user.
Click Done.
By default, the user will also be added as a member. If they only need to be a manager for the site, uncheck the "Also add as member" box.
Click Save and apply.
Remove a site manager
Click your organization name on the lower-left corner of the page and select Sites.
Click the site or level name.
Select the tab Managers.
Check the box next to the user's name and click Remove from site on the lower-right of the page.

In the pop-up window, click Remove from site. By default,the user will also be removed as a member. If they need to remain in the site as a member, uncheck the "Also remove as member" box.