What is the best option for 4 jet-lagged adults to transfer from Munich Int'l Airport to the downtown core?
To find the S-Bahn station, look for the green circle 'S' signs. The station is under the MAC, the building between the terminals.
When you enter the S-Bahn station, there will be a track on either side of the platform. The one on the east (left looking in direction of the train tunnel) is for the S8. The one on the west is for the S1. The S8 gets into town faster, but, for most downtown stations, not fast enough to warrant the extra time waiting for it. Take the next train leaving. If you are getting off east of Marienplatz (Isartor, Rosenheimerplatz, Ostbahnhof, etc) the S8 will get you there sooner.
I have some information about ticket automats on my website.
For four of you, you will want to buy a "Partner-Tageskarte Gesamtnetz" ticket (probably the fourth yellow button down on the right hand column of buttons); it will cost €18,80, less than single trip tickets. It will cover all of your (up to 5P) transportation on all of the conveyance (U-/S-Bahn, trams, buses) in the Munich metro area for the rest of the day.
If you use your credit card to buy a ticket (not sure it will work), you will get two "printings", the credit card receipt (1st) and the ticket. Don't just take the first thing that comes out and go.
The ticket will probably have the day of validity printed on it. If not, it will show "Hier entwerten". You must insert that end of the ticket in one of the yellow (?) boxes to stamp it with the date and time.