I have flown into most of the Paris Ga fields.
As Alex says St Cyr is probably the nearest to public transport. on exiting the airfield walk up the hill (5 mins walk) and the main line train station is behind the buildings at main road intersection at the top of the hill.
As for the airfield itself there was nobody on ATC so just joined as per the arrivals procedures. Note don't get to close to Versailles as thats a Strict no go area. When we came back a few days later somebody was in ATC and they gave us some forms to fill in for Landing and parking fees to which Airports De Paris would send us an invoice to pay, but no invoices ever received. I have heard that nowadays they have been sending invoices out..
Toussus probably the biggest of the Paris Ga fields. Weird opening hours due to local Nimbys. and a long arrivals procedure. Need taxi to nearest railway station.
lognes Eastern side of Paris you need to swing out wide to come around CDG zone to arrive here. Again taxi required to nearest railway station.