In order to add internet guest user [IUser_servername] to the domain one need to perform below mentioned steps:
1) Open IIS Manager.
2) Click on websites.
3) Right click on website for which one need to add internet guest user.
4) Click on permissions.
5) Click on add button from group or usernames.
6) Click on locations. It will ask for network password. Please cancel it.
7) Select the root directory and click on ok. [Root directory should be server name].
8) Click on advance button. It will open new window.
9) Click on Find Now. It will display all the users of the server.
10) Search internet guest user [IUSER_servername].
11) Click ok upto finish.