Does anyone know of a reliable company that would drive 6 people from hotel to airport
Here is the Arlanda Express train site. There are discounts for small groups (2 to 4) traveling together Thu-Sun. And advance purchase discounts up to 45% for nonrefundable 90 day advance purchase. So there are ways to knock down the walk up price of about $210 for 6 adults.
https://www.arlandaexpress.com/fares.aspx
There is also a bus for about $12.50/P.
Last month we used both Taxi Stockholm and Taxi 020 from Glama Stan to Arlanda airport. They both had good sized vans that should fit 6 without problems. They both also have a fixed rates to the airport. I believe it was 520SEK. Have your hotel call the Taxi ahead and I'm sure taking 6 people shouldn't be an issue.
Taxis are not regulated in Stockholm so it is important to use one of the reliable companies. Taxi Stockholm, Taxi 020, and Taxi Kurir are all reliable. Any of these companies can give you a fixed price to the airport.