possessive pronouns - Mass of people its/his/their

In a phrase where the subject is "mass of people", should I use "its"?

e.g.

A mass of people and its flag.

Mass is an abstract word, so it sounds strange to me to use their; but I'm not sure.

EDIT:

Seems that according notional agreement I should use their

but

If the mass of people act like a cohesive body, e.g.

a mass of people is/are united in its/their flag

Is this a situational agreement and is the use of plural forms wrong?

Could you help me?