How To Create Groups, Add Users In That Group And Give Permissions

SharePoint site security levels are assigned to Groups/Users. Users are added in different groups and each group is assigned security levels.
Security levels are assigned by the administrator who is adding the user. There are four levels by default.

*  Full Control - Has full control. 
*  Design - Can view, add, update, delete, approve, and customize. 
* Contributor - Can add content to existing document libraries and lists.
*  Read - Can view pages and list items and download documents. 
*  View Only - Can view pages, list items, and documents. Document types with server-side file handlers can be viewed in the browser but not downloaded. 

To add Groups in SharePoint site follow the following steps:

1. Go to your site and login with administrator. From Site Action choose Site Settings.

2. From Users and Permissions select Site Permissions and from ribbon click on  Create Group.

3. Give a name to user Group and choose the permission level. Click on save button.

 

If you want to change the permission level of existing Group, choose Grant Permissions in Step-2 and follow the following image and choose the permission level.

  

Add Users In Groups

4. Go to Site Action and Site Settings as in Step-1

5. From Users and Permissions select People and Groups and from left panel choose your Group in which you want to add users.Click on New->Add Users.

6. From pop-up window insert user name and check for existence. If user exists click OK button.