Invitations to sign up for these three sites will be sent by email to members of the Church in stages throughout 2014. Emails will be sent first to members with family history callings, including family history consultants, family history center directors, stake indexing directors, and high priests group leaders. Then emails will be sent to other members in specific areas.
When you receive your email invitation, it will explain how to register for your personal account. The invitation e-mail will only work for the member who receives it.
Update June 5, 2014: This post previously recommended that you change your name and email settings in FamilySearch.org in preparation for receiving your invitation. I have confirmed with FamilySearch that these steps are not necessary.
Update August 11, 2014: If you haven’t received your invitation yet, try FamilySearch.org/partneraccess.
For more information see LDS Member Access to Partner Family History Websites.
Update (February 18, 2015): You can now also get free access to AmericanAncestors.org,.