You must pass the users Infusionsoft Id in the link to autolog them in. (Its also required that you include your own custom Auth Key when using this feature.)
https://keap.memberium.com/?memb_autologin=yes&Id=6&Email=email@domain.com&auth_key=yourauthkey&redir=/test-page/
How to auto log in someone from an Infusionsoft email template:
Include this for the url in your template –
http://yourdomain.com/?memb_autologin=yes&Id=~Contact.Id~&Email=~Contact.Email~&auth_key=yourauthkey&redir=/silver-video-2/
How to auto log someone in from customer hub
http://yourdomain/?memb_autologin=yes&Id={{user.infusion_id}}&Email={{user.email}}&auth_key=yourauthkey&redir=http://yourdomain.com/video-2/
Example Auto Log In with Redirect:
<a href="http://yourdomain.com/?memb_autologin=yes&Id=~Contact.Id~&Email=~Contact.Email~&auth_key=yourauthkey&redir=http://yourdomain.com/silver-video-2/">Log In Automatically Here</a>
Example Auto Log In with Redirect and Authorization Key (*Set Auth key on your options page):
https://keap.memberium.com/?memb_autologin=yes&Id=6&Email=email@domain.com&auth_key=yourauthkey&redir=/category/shortcodes/