Joining an existing organization is free, but accepting other users in your organization is not available in the free tier. You need a paying subscription to enable it. You can join an organization by clicking the "Join Organization" button in the home screen.

There you can search for the organization you wish to join, either by name or by it's unique id:

Once you have found the desired organization, you need to send a join request. This will notify an administrator of the target organization, who then has to approve your request. As soon as that has happened, you will be able to select the organization from your home screen and collaborate with other users in that organization.

Accepting Users

If enabled, and you are an organization administrator, then you can accept (or decline) join requests. If you decline, you can also pass a message to the requesting user for more feedback. When a user has joined the organization, (s)he can view all repositories configured on the organization.