When you right click on the folder and select the option Permissions, a pop-up window will open.

In the window, the current permissions for that folder are displayed. If you have write permissions, you can add or edit user permissions.

Permissions can be given to a group of users as well as to individual user.

If you want to add 

permissions to a group of users,

choose Invite group option and then

enter the name of the group/role, and select Write or Read.

When you have finished, click on the icon Add, and after that click Save. This will add new permissions to a selected group/role