My answer was accepted without the में.
I chose sath sath mein and it was accepted.
I would like to know the subtle differences between "together":
Although both mean together, EK SAATH is used when objects/ items of different types are being referred to, while SAATH-SAATH is used when similar items are referred to.
एक साथ is not accepted
Seems like the feminine should work since the "people" could just as well be all female