I want to know are all the girls/women mahrams of/to each other? If yes, then can a group of girls travel on their own without a male-mahram?
A 'mahram' in the marriage context , refers to being "permanently unmarriageable". so the husband's brother is not mahram to his brother's wife even-though the marriage between them is haram and invalid (because they are only conditionally unmarriageable ) .
Also mahram refers to one of the men a woman should travel with, and that she can be with without hijab , that's why here a woman is not considered mahram to another woman . So , the husband is mahram to his wife because she can travel with him alone and he can see her without hijab of course.
a mahram is used for men and women. For example, the daughter is mahram to her father , etc.
In travelling, a woman cannot be a mahram to anybody.
The concept of Mahram in Islam applies to two or more persons of opposite sex. Generally, mahram of opposite sex who are prohibited to marry each other by the Nobel Quran mainly because of blood or marriage relationship. daughter vs brother- father, grandfather, uncle etc. son with mother, aunt, grandfather etc. The marriage prohibition law is also found in all secular and non-secular countries with the slightly different Quranic injunction.
A person to whom a marriage to (opposite-sex) is possible they called non-Mahram (strange) to each other. thus prohibition of meeting alone or traveling apply
so two or more girls meet each other in private or traveling are perfectly alright
Generally, Muslim men or women are not permitted to be alone with a non-Mahram person without a valid reason.
Traditionally the women are not permitted to travel (long distances) alone or with a non-mahram person. A Muslim woman must be accompanied by a Mahram person (husband, brother, son, father), For example, going for Hajj - some scholars now give permission (with some conditions) to Muslims women without a Mahram for Hajj because of its obligation in nature