You can take the magic kingdom (MK) express monorail to the ticket and transportation center (TTC) (express runs only from MK to TTC)and then get on the epcot monorail, you could take the resort monorail from magic kingdom and still get off at the TTC and get on the epcot monorail, but you that will take a bit longer. You can also take the ferry from the MK to the TTC and then get on the monorail to epcot.
To get to MK from the parking lot (TTC) you will either get on the express monorail (quickest, normally) or the ferry from TTC to MK or the resort monorail to MK (but it will have stops along the way).
and yes, offsite guests can use all the WDW transportation..