Djurönäset Conference Centre is located on the Djurö island in the Stockholm archipellago.
Getting at the conference centre from the airport is not that difficult. You have two options.
1/ Taxi.
Värmdötaxi has cars for 7 people (1870SEK) or 4 people (995SEK). If you want to pay this price, you can team up and order such a taxi. I advise you to book in advance through this webpage. Beware that taxi market is not regulated in Sweden so you should negociate price in advance (or book online).
2/ public transport
You first need to get to Stockholm city. Either by bus (99SEK each if bought online) which take almost one hour, or by train (280 for 1, 300 for 2, 400 for 3 or 500 for 4 persons) which takes 20min.
From T-centralen (the main station) you need to buy a ticket ABC-zones (72 sek). Either you can buy it at the machine (but not all machines are able to print) or you can buy it through the app just before you take the metro (Google or Apple store). You need to take the metro to SLUSSEN (either red line direction Norsborg or Fruängen or green line to Farsta strand or Skarpnäck in all cases it is only two stops). From Slussen follow the signs to the bus terminal and take bus 434 or 433 (timetables available here) until you reach Djurönäset. The conference Center is just by the bus stop.
