Access via email works for signed-up users only, so make sure the person you share the tag with has an account at UXPressia (they don't have to be on a paid plan).
To grant access to someone via email:
Go to the Tag page.
Open the sharing popup. In the form that pops up, type in the email of the person you want to share the tag with. Specify their level of access: view & comment/edit. You can add a short message if needed.
Click 'Invite' and that person will get an email with the invitation.
The shared tag will then be available under the user's 'Shared with me' tab on the left panel of the dashboard. Keep in mind that users with the free plan can share tags with just one user.
And, of course, you can unshare the tag at any time: