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.