About sharing groups
Sharing groups let you save the permissions you want a user or group of users to have so you can reuse the permissions for other items. You can create sharing groups for individual users or for all users in a particular course or group. You can have multiple sharing groups for the same users that you apply in different situations. For example, you can create sharing groups named Comment, and Edit and Review, and share them with the same group of peers. You would use the first sharing group for items you want to collect general feedback on, and the second sharing group for items that you want others to collaborate on or revise in detail.
An organization, department, or course can also set up sharing groups and share them with you. Sharing groups that are shared with you are either optional (you can choose to apply them to specific items) or automatic (you have to use them). Automatically shared groups are either visible (appearing in your Sharing Groups area) or hidden (information on the group is not available to you).