Click on the option to manage your projects as shown in the image.

Select the project and press the pink plus icon.

Type in the email address(es) of the user(s) you want to invite to the project. 

Click on preview once you are happy to move forwards.

Select the permissions that you would like to provide to the user(s).

Click confirm when you are happy to move forwards. Congratulations, you have now invited an external user to the project!

Providing Users External to Your Organisation Access to a Project