The most you could get in an Agile would be 4 people with a back sofa providing two with full lap and shoulder seat belts. A middle sofa seat in a lot of rear sofa/beds have just a lap belt. Few provide head restraints with an across the back sofa. A lot of Sprinter Class Bs have two passenger seats behind the cab plus the rear sofa with seat belts. Full size Sprinter Roadtreks and Airstreams are the two to check out. With 5 or 6 people in a B you would definitely have to tent camp. I would question whether the waste tanks could handle that many people or even if you could find enough storage for tents, sleeping bags, clothes and personal items especially for a cross country trip.
Anything can be done I guess. One of my earliest memories was a two day vacation trip (and back) from Indianapolis to Tampa Bay in a 1955 Chevy station wagon with four adults and five kids.
2015 Advanced RV Ocean One Mercedes Benz Sprinter
Previous Class Bs:
2011 Great West Van Legend Sprinter
2005 Pleasure-way Plateau TS Sprinter