VORTIME,
The flyer forum and PPLIR both have people who fairly routinely offer a spare seat for other pilots to experience the trip. Often it is a business trip that the pilot is flying alone and is therefore happy for company. There are also flyins posted where people are offering spare seats. There is a group of aircraft planning to go to Morocco this autumn for example on Flyer.
The North Atlantic crossing is very different from European flying (like no one to talk to, no radar coverage, no land, limited alternative fields, long straight lines, etc.)