As stated in my comment there's a concept in fiqh of the so called trustworthy group in travels. I'll try to explain it based on the few information i could gather. Unfortunately about all of my sources are in Arabic.
On the View that traveling without a mahram is not allowed
The Hanafi and Hanbali schools of fiqh don't allow a woman to travel alone without a mahram based on the sahih hadith, here just one of many similar statements (and from different Hadith collections) from Sahih Muslim:
It is not lawful for a woman believing in Allah and the Hereafter to undertake journey extending over a day and a night except when there is a Mahram with her.
And in Verse (3:97) it is said that:
And [due] to Allah from the people is a pilgrimage to the House - for whoever is able to find thereto a way.
the expression: able to find thereto a way is was also translated those who can afford the expenses (for one's conveyance, provision and residence) or who can afford the journey (See for example translation of Muhsin Khan or Yusuf Ali).
Imam Ahmad supported his view by explaining that a mahram is one of these afforded requirements for the journey.
On the View that traveling with a trustworthy group is allowed
The Maliki and Shafi'i school of fiqh is allowing the travel of women in a trustworthy group (or secure companionship) the so called rufqah al-ma'munah الرفقة المأمونة.
Note that even among the salafi or hanbali school the so called sheikh al-Islam ibn Taymiyyah accepted this kind of company and from the modern scholars Salman Fahd al-'Awdah (Saudi-Scholar) has declared this in a fatwa dated on October 16th 2012:
"يجوز للمرأة أن تحج مع الرفقة المأمونة إذا لم تجد محرماً، واختاره ابن تيمية وقال إنه متوجه في كل سفر طاعة، وفي المسألة خلاف مشهور"
(My Transalation take it with care):
it is allowed for a woman to travel (for hajj) with a trustworthy group if she can't find a mahram. This is the choice of ibn Taymiyyah who said: This is the case for any travel for a compliance, and there's a well known dispute (among scholars) on the matter.
According this statement it is allowed for a travel to perform a worship like hajj or 'Omrah, but not for a travel for tourism etc..
What is a secure companionship
Now maybe i need to explain what is a trustworthy group or secure companionship: This is a group of pious and righteous women and men, even if none of them is a mahram to her.
A major point we may need to discuss are such people existing these days? Therefore the practice I know about is that at least one mahram of one of those women would be part of this group, and even that could be a problem therefore in addition to the quoted condition of ibn Taymiyyah the scholars of the Maliki and Shafi'i school added, that it is not allowed for a young and beautiful women or a non pious or non chaste woman. In those cases scholars wouldn't allow them to join a trustworthy group because preventing an abuse or degeneration is put in front the interests.
Here is a salafi fatwa defending the view that travel without mahram is haram.
Other sources (in Arabic) defending the view of the trustworthy group under the given conditions: 1 and 2.
Note that the tendency on islamweb is to allow travel with a trustworthy group, even if some of the scholars there are also salafi.
EDIT: I just found this fatwa in English which may help you a lot.
And Allah knows best!