Identify users from login

Hi Guys,

Hopefully someone can give me a little advice on this ( @Sanford Whiteman​ ).  My company are wanting to put a login on their website that will allow users to use their products as well as browse the website (tracking everything they are doing).

Once the user logs in they want to not show them any forms on the website, basically they want to have the "If known user" functionality automatically working. Is there a way to do this using API or cookie manipulation etc... 

Besides filling in a form or clicking a link in an email I'm not sure how we can let Marketo know which user is currently on the website.

Thanks,

Gerard