It is a very powerful technique to have a central table of users where you can pull in centralized employee "attributes" into other apps.
If you change the key field in your employees table to their email address, then you can make a field on the other apps which will calculate the current user's email address and then that field can be used as the "reference" field on the right side of a cross app relationship.
[Current user email address] formula would be
UserToEmail(User())
The User() function returns the userid of the current user and then that gets converted to an email address.