Security within SharePoint 2013
Granting permissions to an individual
By assigning people to SharePoint 2013 groups, you can see what permissions an individual has and assign those same permissions to someone else simply by adding that person to the same group. Occasionally, however, you might need to assign permissions directly to a user to grant access to a site or its individual items.
Grant permissions to an individual
On the Site Settings page, in the Users And Permissions section, click Site Permissions.
On the Permissions tab, in the Grant group, click Grant Permissions.
In the Share dialog box, in the designated text box, enter names, email addresses, or the word “Everyone”.
Click the Show Options button.
In the Select A Group Or Permission Level list box, select a permission level.