Send employees to you custom sign up page. Admin can access your customer sign up page HERE (*.goodseeker.com/company/settings/authPage)
Invite individual users. An invited user's name will show up as an option to be added to a story before they accept the invite. (/company/settings/users)
Bulk upload users. All added names will show up as options to be added to a story before they accept the invite. (/company/settings/users)
Other ideas: Add a button to your intranet, link in a slack channel, link in a newsletter, etc.