Before you begin you will need to review Setting up Anonymous Registration.
You will first need to log into your Root Site as an Administrator and add a new Role by going to Site Settings/ Extranet Roles.

In the top ribbon select Roles then select New Role.

Then name your new role, and click save.

The new role should now be listed in the roles.

Next (still within the Root Site) you will need to go to Site Settings/ Extranet Settings

Click Security policies

Click Security Policies in the top ribbon, then New Policy.

You will then need to add the Role name to the Security Definition section. Once the People Picker has found the Role name, select Domain Name within the Field Type section. Type in the Domain Name Match Expression within the Security Policy Settings section then click save. Example: acme.com

Log into the site, that the domain name user needs to access, as an administrator. Select Site Settings then People and Groups.
(In our example the users needs to register for access to: extranet.awbikes.com/sites/acme)

Select the appropriate Security Group. In most cases the Visitors group is selected. Next, select the NEW tab and select Add Users to this group.

Within the “Add people to the group” type in the full name of the role created previously and select the name from the list.

Click Share.

You should now see the Role/Policy in the Group.

Once you have your Policy in place, anyone that browses out to the site and registers using the appropriate domain name, will be automatically added to the correct role/policy. Once in the proper security group, they will be granted access to browse out directly to the site and log in.
If the new user clicks Sign In at the time of registration or from the approval email they will be taken to the Root Site. You can add a Web Part to the Root site that will show a directory of links to all the sites they have access to. I have attached a link for the instructions for setting this up.
http://excm2013r2.premierpointsolutions.com/extranet-directory-web-part
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