Crew is an activated ability. Suppression Field makes it so Crew N will cost you tapping any number of creatures with total power N or more and paying 2.
EDIT:
2nd edition: is the tax on the vehicle or on the creatures being tapped
Crew is an activated ability of the vehicle, which cost is to tap creatures you control. The creatures you tap don't have abilities of theirs being activated, so you only pay one tax of 2 for the vehicle. That you're not tapping the creatures for an ability of their own (and that there is no tap symbol involved) also means you can use creatures affected by summoning sickness for this, by the way.
The main rule for Crew as a reference :
702.121a Crew is an activated ability of Vehicle cards. “Crew N” means “Tap any number of untapped creatures you control with total power N or greater: This permanent becomes an artifact creature until end of turn.”
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
EDIT: Adding card in question suppression field
2nd edition: is the tax on the vehicle or on the creatures being tapped
(Srry for multiple edits on phone not computer)
EDIT: Crew is an activated ability of the vehicle, which cost is to tap creatures you control. The creatures you tap don't have abilities of theirs being activated, so you only pay one tax of 2 for the vehicle. That you're not tapping the creatures for an ability of their own (and that there is no tap symbol involved) also means you can use creatures affected by summoning sickness for this, by the way.
The main rule for Crew as a reference :