How do I mark a company on Google Maps, so it could be added to contact page via iframe?
The Google Static Maps Wizard makes it easy to mark a location in a map & embed it in a web page. Google Static Maps API accepts inputs to generate a map through querystring values.
Google's Driving Directions gadget shows a map & also provides driving directions based on the address of your company's location that you can preset & the starting point specified dynamically by a customer.
The first thing to do is to add your company to Google Local through the Google Places interface http://google.com/places This way people will find it not only graphically through a user-defined placemark (which is actually what you were asking about, but really not what you want to do!) but also by searching Google.
See also Google Places for Business http://places.google.com/business
And once it is added, you may use the embed code to add it to your website. You don't necessarily need to add it to places to have a marker and map added to your site (but I agree with msanford that's what you should do). Simply, you can search [address (business_name)] add then use the embed code for that result.