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 on a paid plan).
To grant access to someone via email:
Open up sharing dialog either by either by clicking on the SHARING icon on the project’s bar on the dashboard...
… or hitting the SHARE button on the project page.
In the text field right under 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 on the dashboard's left panel.
Keep in mind that users with the free plan can share projects with only 1 user.