You need to be an administrator of 2c8 Portal to do this.
If you have an external user with their own license for 2c8 that needs to access your material, go to the section Users. Click the button Invite external user and enter the email of the person that will need access to your work. This person will get an email with an activation link to your portal. After that, you need to make sure that the external user gets access to your server and the correct repositories. You can accomplish this by adding the external user to a group that has access to the correct server, repositories and has suitable permissions. You can also choose to add the external user manually to the correct server by navigating to the Server section. Click on the correct server and add the external user. After that, you will need to open 2c8 Modeling Tool and add the external user to the correct repositories.