Any user who has "Admin" status within an organisation can add (or remove) users from your organisation.
To do this, navigate to your profile, in the top right hand corner of the screen, and then select "My Organisation":
Once you have done this, you can simply select "+ Add staff member to organisation":
You can then enter their email address to invite them to join your organisation:
Users should always be conscious that if they leave one organisation to join another, they will lose access to all farm accounts which were in their previous organisation.