Security within SharePoint 2013
Customizing SharePoint groups makes it possible for your organization to effectively grant access to only the users it specifies. When you create a group, you can name it, assign it a permission level, and add people to it.
Create a group
On the Site Settings page, in the Users And Permissions section, click Site Permissions.
On the Permissions page, click the Permissions tab and then, in the Grant group, click Create Group.
On the Create Group page, in the Name text box, enter a name for the group.
In the About Me text box, enter a description.
Assign a group owner to the group.
In the Give Group Permission To This Site section, select a check box corresponding to the permission level that you want to assign to the group.