You can use the Quick Actions wizard to grant selected users or groups custom permissions for entire sites. To do so, proceed as below:

1. Navigate to Explore Sites and click the Quick Actions button:
   
   
   
   
2. Select the last option in the wizard: Set Site Permissions, and click Next.
   
   
3. Select your Organization, and then the site you wish to modify, and click Next.
   
   
4. Select the user / group you wish to modify permissions for, and click Add User or Group, they will then be added to the table below.
   Repeat this step to add as many users / groups as you wish to modify.
   
   
   
   
5. Select the desired permissions for each user / group listed. You have the following options:
   • Owner: The user or the group becomes an owner of the element.
   • Edit: The user or the group is granted write access to the item.
   • View: The user or the group is granted read access to the item
     
     
6. Once finished selecting permissions, click on Set Site Permissions to apply the changes.
   
   
7. A summary of the performed actions will be shown, and then click Done to complete the wizard.

Note: All actions will be audit logged and viewable in the Activity Log, along with the details of the user account that performed the action.