How do I set up surveys so that everyone of our employees, but only them, can access the surveys?

I would think this is as server issue rather than a Limesurvey issue - httpd.apache.org/docs/2.2/howto/access.html

You could define one token that everyone must type in, and set "allow multiple responses" to true (and Enable token-based response persistence to false).
Or if you do'nt want to use tokens: let the users type an password on the first page, and hide all groups if an incorrect password is typed in.
Frans Marcelissen, DigiPsy