It's very easy to add new users to a project.
To add new users to a project:
- When the project is open (you are viewing the question list), at the top of the screen click share.
- The list on the left shows who currently has access to the project. The list on the right shows the list of users who may be given access. In this list, click the users you want to invite.
- Click invite.
Note: the actual permissions on the project (i.e. which aspects of the project are visible and can be modified) depend on the user's role.
You can also share a project with other users from the Project list, as explained here.