If the user loggs in or you have user information, this means you have contact_key for that user. You can set contact_key in order to match user with the browser. There are two functions for getting and setting contact_key.

setContactKey

If user logged in set user id. This is important for identifying your users. You can put this function call in every page. It will not send unnecessary events.

dengage('setContactKey', userId);

getContactKey

If you need to get current user information from SDK use this function.

dengage('getContactKey', function(userId) {
     //use userId (contact_key) here
});