In our state, people arrange parties in hall located in the mosque. Are women allowed to go the iftar party in mosque during menstruation?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If the hall you mentioned in your question constitutes an extension to the mosque (i.e. not specified for prayer), then there is no harm for a menstruating woman to attend the Iftar party in it (hall).

If this hall is specified for prayers, then it is impermissible for a menstruating woman to stay in for Iftar or anything else.

The Prophet said: “I do not make the mosque lawful for a menstruating woman or a person [whether a man or a woman] who is a state of Janaabah (i.e. in a state of sexual defilement).” [Abu Dawood]

Allah knows best.