Margaret and Helen go to a dinner party discussion group, at which they debate the question of donating money to the poor. Margaret asks the businessmen in that/this party to give poor people a huge sum of money so that they can be able to face the vicious challenges of life.
I wouldn't use either. The preceding sentence is sufficient to establish that Margaret is asking at the same party. So, the sentence should read, "........asks the businessmen to give........." But if you insist on "that/this", or even "the", for a narration, any one would be OK.