Imam Abu Hanifah (RA) has a very strong opinion on the issue of women traveling without Mahram. Therefore in Hanafi Mazhab, a woman should not travel alone or with a group of women or with a person who is not Mahram. However, according to Imam Shafeyee, women are allowed to travel in groups for the sake of importance of Hajj. Perhaps many Hajj organizers are following the opinion of Imam Shafeyee nowadays.
This is not a matter of Hajj, but a Shariah ruling on the matter of travel by women. The Hajj of a woman who travels without a Mahram will still be valid if performed correctly. However, it is highly preferable, that for the purposes of Hajj, a highly auspicious event and one of the five pillars of Islam, care is taken to follow Shariah rulings in the best possible way.