Go to the Members page and click on + Invite Member. Now, put their email address and select the projects you want to invite them to. (Or after joining members of your organization, you can assign projects to them as well).
Alternatively, you can also click on See more > Invite Member.
Now, put their email address and select the projects (not mandatory to select the project here) you want to invite them to.
Now click on Send & Add Another if you want to invite more people or if you are finished inviting for now, click "I'm Done With Invitation".
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).