Grant additional permissions to users or groups in a target location.
Target the item(s) you would like to add permissions on from your Explorer tree.
Select Add Permissions from the Quick Actions menu in the right pane, under Security.
You will now see the Add Permissions page.
This table displays the targets of the assigned permissions. Please use this information to confirm that you are assigning permissions in the right location.
Select the users or groups you want to add permissions to.
Start typing the name of the user or group, and select them from the drop-down
Note: You add either users or groups in a single operation, not both.
Assign permissions to the selected users or groups by adding them to existing groups, or by giving them explicit permissions.
Select the action from the drop-down.
Add to Existing Group: Lists all the groups within the target location that you can add the user(s)/group(s) to.
Assign as Site Collection Administrator: Grants site collection administrator permissions to the user(s), or to all users in the selected group(s). Sits collection administrator permissions give users full control over all sites in the target site collection(s).
Add Explicit permissions: Assigns specific permissions (such as design, edit, contribute, and read) to the user(s), or all users in the selected group(s). You can select if you would like to apply the permissions to all subsites, lists, and libraries with unique permissions, and to list and library content.
Once you select your action, click Apply.
You will now see your results page (any warnings or errors, if applicable, will appear on a separate tab).