Maid Marian

Maid Marian was not an origonal character in the Robin Hood tales. She was added during Victorian times to add a more 'romantic' feel to the Robin Hood tales, and make it more palatable to the general audience.
As far as Little John and Will Scarlet go, there are no veritable reasonings for them except to add a sense of comradery to the tales.
The addition of Friar Tuck is possibly a move meant to further the idea that Robin Hood has a religious side. In the modern tales, it was often pushed that Robin Hood loved Maid Marian second, and the Virgin Mary first. Such a 'fact' of Robin Hood being religious was also added sometime around the Victorian era to make him seem more of a noble hero.
There are many theories behind the modifications of the Robin Hood tales, as early as the 1400's.
Robin symbolism