If groups are more fun then solo play, then no bonus is needed or appropriate. It being more fun is already sufficient motivation.
As Syasob noted, this would create a situation where, to maximize reward, you'd run multiple fast missions to the multiplier up, only then to run you real intended mission. So, 6 captures to raise the multiplier, then the mission you actually want to do.
And then you have the practical issues of how the mechanics actually work. If a 4 man group runs 5 missions, then one leaves and the rest run another mission, are they getting the bonus for those 6 mission still? What if 3 people leave, does the remaining one still get a bonus? If so, then shouldn't solo players get the bonus - they stay with their 'group' of one? Does a host migration cause a bonus reset?