Because Survivor is elimination based, the system will automatically consider them eliminated since they don't have picks in for previous weeks.

You can add them manually by going to Commissioner Console --> Member Management --> add/copy members --> Add member

From here you can add them by using their username (if they have an account) or creating a new one. Once added you can go in and add winning picks (or issue mulligans) in Member Pick Maintenance until they are caught up and can make their own selections for the current week.