To make things easier, employees are able to sign themselves up as Advocates. This option can be turned on and off within the Board Settings tab's Sigup options:
Once the Inviting option is turned on, you will be able to share the sign-up link with users from your company and have them join on their own. Note that users who join this way will still count against your license limits.
Note that you would need to enter yourcompany.com to only allow email addresses with @yourcompany.com in them.