It's not possible using just an .htaccess
file as all visitors would be presented with an HTTP standard authentication dialog if you enabled it on your domain.com site at the doc_root
level.
Without knowing what scripting language you're using? (you've not indicated in the tags, just apache), but you could provide one index page that both acts as a landing page for users/potential-users as well as provide a login (username/password form) for MFP parties (wherever they may come from).
That way, you fulfil both needs without offending or discriminating in any way against any party.