What are the options to getting to Mandal for 6 people with luggage?
If you are staying in Mandal just a few days, I would rent a van and drive. Book in advance. Rental companies at Kristiansand Airport: http://www.avinor.no/en/airport/kristiansand/tofromairport/Car+rental
Distance is 60km, driving time 1h.
With public transport, airport shuttle to Kristiansand, time table:
Transfer to local bus to Mandal, time table: