Adding others to your J1 account/organization is done using a simple invitation process. To send an invite:
Go to Settings, select Users & Access.
In the user groups modal, click Add Group and enter a name for the new group and, optionally, a description.
Click Add User and enter the email address of the person you want to invite.
Click Send Invitation.
The user receives an invitation email to join your account/organization and is prompted to accept the invite upon login. New users are then prompted to create their J1 user account.
You must be a member of the Administrators group to be able to invite other users.
If you are an Enterprise customer and use SAML SSO, see the instructions here.