Why would anyone join a polygamous group?

I was just reading a book on people in polygamous groups. Any way its about a girl who is forced to become a teenage bride, to her first cousin! In the beginning the author writes about how her fathers first, wife( of the three had had ( at the same time(plural wives, holy crap) - the second being her birth mom- she willingly choose to join a polygamous group after knowing that as a women she would be considerd a lesser being and would have to marry who ever the priest chose for her, and have to share her husband, (boyfriend at the time)with other women and have to deal with other hardships. All in the name of a religion that was filled with lies. For example they thought the world would end in the year 2000, and god would pull them from the destruction, and kill all those who were “wicked”, and when it didnt happen the priest made up some bogus lie about how god gave them more time. They also think that when a women goes from one husband to another the dna of her children changes to match those of her new husband. I am just curious as to why some one would by into that and still decide out of free will to join?

Btw: the book is called stolen innocence,by elissa wall and if the writer is on the site I would really like to have some questions answered.

Thank you