the left and right tables.

I am not an expert on hibernate so please be gentle with me.
I have a java application where I have 3 tables.

One is a staff table with demographic info.
One is a group table with group_id and description.
a third is a table in the middle called staff_group that contains
staff_id (from staff table)
and group_id ( from group table)

in my staff entity I have a field as such List<group> groups = new ArrayList<Group>() which will contain
all the groups that an individual staff member will belong to.

the group table is pre-populated and should not be changed at all.
However, when a staff member changes a group such as remove or add, how do I get my java code
to only update the join table that holds the relationships between staff and group?

Thanks

Eric