As far as I understand the grouping functionality is hierarchical only. Is there no way to create a graph such that some nodes belong to more than one group. For example:
Nodes: a, b and c
Edges: a->b, b->c, a->c
Groups: X, Y
Group membership: a \in X, b \in X, b \in Y, c \in Y
If draw the above, group a and b into group X all is fine (as expected). If I then try to create a group including b and c, ie: b is in both groups, then b is removed from group X leaving me with two groups X and Y, but b is in Y not in both X and Y.
