Go to the Members page and click on + Invite Member.

Alternatively, you can also click on See more > Invite Member.
Now, put their email addresses. For multiple users, use a comma or a space. You can choose projects/teams for them to be assigned in (optional now). Then, click on the "Send Invitation" button to send the invites.

After this, your employees will get an invitation mail to join the organization. S/he will put in their name and password to sign up (email should be automatically detected).
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Use "Upload a CSV" button at bottom, to upload a CSV file