Logins aren't syncing across Availability Groups

Question

We have 2 servers in an AlwaysOn group.

While the user accounts within each synchronized database exist on both servers, the database instance level logins only exist on one of the servers. Ie DBINSTANCE->Security->Logins are missing on one server.

Therefore when there is a failover, I get login failures on the second server (which doesn't have the corresponding instance level logins).

How do I overcome this issue? Was I supposed to set up the user account in a special way?