Sort people to groups (similar to min cost flow network)

Im having trouble with sorting people in to groups.

What I have to do is sort people (around 100) to groups (8 groups). Each user has his prefered groups ranked from 1 - 6.

I need to sort them with the best possible outcome for each day of the week (monday to friday - 5 different arrangemets). User should never be in the same group twice but it can be in group that is not on his "preffered" list (if other outcome is not possible)

Similar problem is described here: Sorting People into Groups based on Votes but i dont know which library should i use for my project and how to make 5 different groups I'm using node (javascript).