When joining a domain, you are sending a request to the domain’s administrators to add you as a member of their domain. Below are the steps on how to join a domain:
Domain user joins a domain:
  1. Click My Domains from top navigation menu.
  1. In My Domains page, find the search box that allows you to search for a domain.
  1. You can choose to search by Domain Name, Company Name, Description or Anywhere. Enter the value for the domain, select from the filter and click Search.
  1. Find the domain that you wish to join and click Join under it.  The Join function is shown only for those domains available for you.
  1. A confirmation message stating that a request to join the domain has been sent to the domain's administrators will be displayed. You have to wait until a domain administrator accepts your request.
NOTE: A message informing the domain administrator about the join request will be displayed in the home page the next time the admin logs in. 

Domain Administrator accepts your request to join the domain:
  1. While logged in as the domain administrator, click on My Domains.
  1. Select the domain and click Manage Domain.
  1. Under the section Pending join requests, find the user request click Accept.
  1. The domain user is now added as a member of the domain.
  1. Domain user will see that he/she is now a member of the domain as listed in My Domains page.