Under the menu Users, you have an option called Invite Users.
Simply add the email address of your colleague or external partner that you want to give access to your organization.
This person will then receive and email with a link that transfers them to a signup page. If the email hasn't arrived within a couple of minutes, have a look in the spam box - It's not our fault, your spam filters are too good!
What if it doesn't work? Also read Troubleshooting: Invite a new user.