To use xpJoin, either drag the objects to be joined into the Objects list, or make them child objects of the xpJoin object, or a mixture of both.
In this scene, both the Sphere and the Cube have been dropped into the Objects list joining them. An xpCollider tag is attached to xpJoin, so that particles collide with both the Sphere and the Cube.
xpJoin, Object tab menu settings.
As First, by default, the xpJoin can add a Phong Tag to the generated object if desired.
The alternative options are: As Last, Edges and Custom.
If the first object in the list has a Phong Tag, that will be copied to the joined object.
If it does not, the joined object will not have a Phong Tag.