Access via email works for signed up users only so make sure the person you share the project with has an account at UXPressia (they don't have to be Pro users).
To grant access to someone via email:
- Open up sharing dialog.
- In the text field right under where it says "Invite by Email", type in the email of the person you want to share the project with. Specify the level of access - either view only or view and edit.
Click INVITE and that person will get an email with the invitation. The shared project then will be available under the Shared with me tab of the dashboard.
Keep in mind that users with the free plan can share projects with only 1 user.