As u know arjun had won drupadi in the self selection of husband .
after that he took her to his hut and said to his mother, " look mother, what prize have i got?"
then, his mother replied, without looking at the prize ,"whatever you have got, share it among you and your brothers."
then she turned and was shocked to see that draupadi was the prize.
the pandavas HAD to follow her instructions as the mother should always be followed in those days.
so, following mother's instructions, draupadi had to be shared and was married to all 5 brothers.