Share or create unique permissions for a folder

In a document library, hover over the folder and right-click it. Select Manage access.
Select the + beside Direct access.
Enter the name of the individual or a SharePoint group.
- To add multiple users, separate their names/addresses with a semi-colon beforehand and paste it in the field.
- Select the pencil icon to choose the access level.
- You can enter a message and check/uncheck Notify people to send a notification.
Select Grant access.
- If you're granting access to an external user who's system isn't on O365 or encounter an error, use the Share option in the Manage access pane.
- To enable external sharing on the site, contact EASI SharePoint Support.

You can remove groups or individuals this way. Under the Direct Access section, select the pencil icon next to a group or individual and Stop sharing.