Can a woman go to Umrah without a mahram? Can she go with a few sisters?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The jurists are agreed that it is forbidden for a Muslim woman to travel without a Mahram (i.e. a husband or a person whom one is permanently prohibited from marrying due to suckling, marital, or blood relations).

With regard to traveling in order to perform the obligatory Hajj and obligatory 'Umrah, [lesser pilgrimage], the jurists differed in opinion, and the predominant opinion is that she is not permitted to travel at all except with a Mahram, as the Prophet , said: “A woman should not travel except with a Mahram.” [Al-Bukhari and Muslim]

Allaah Knows best.