Yeah, they kinda messed up the group handling in 2.6.
1. Export your scene to jpk.
2. Import the scene jpk. You will get the empty groups (have this 0 object in them) in the outliner group view (like for the collidable and clampv group in the pic):
3. Select one object you want to be collidable. Switch to object editor (box icon) of this object. And click "Add to Group". Select collidable from the dropdown list.
4. Select the other objects you want to be collidable too (shift+select or b), the first object has to be in the same selection. Then press shift+ctrl+g. Select collidable from the dropdown list.
I know it is feels cumberstone, haven't found a simpler way yet.