Dear Houdini

After geometry is packed, create an attribute "collisiongroup" in points.

s@collisiongroup = "col1";

"collisinoignore" can be specified if a packed piece shouldn't collide with collision groups and/or dop objects.

s@collisionignore = "col2";

When you don't want interaction between collision_group_1 and collision_group_2,
collision_group_1 has to have "collision_group_2" and collision_group_2 has to have "collision_group_1" in "collisinoignore". If either of them is missing, both group collide with each other.
Both has to ignore each other.