The Periodic Paralysis Association (www.periodicparalysis.org) has a discussion board that may be helpful in finding other people who experience the periods of muscle weakness (periodic paralysis) that are characteristic of Andersen-Tawil syndrome.
Also, Karen Carr Beihl runs a Facebook page and private Facebook group for those with Andersen tawil syndrome.
Andersen-Tawil syndrome is also listed on Patients Like Me.