If you add new logins to your environment and want the associated database permissions to flow through to replicas you must create the logins with common sid identifiers. To do so query for the sid for the login from the syslogins table. After you have the sid you can create the login on the replicas as follows:

create login <name> with password='', sid=<sid>

After you do this database permissions will flow through from the primaries to the replicas and in the event of fail over users will have their appropriate access.