As ockham notes, groups are handled as separate things.  Usually as body parts in a figure. And the way to 'combine' the geometry is through welds of the figure body parts.  But there is no welding on a prop so you need to have no or one group only.

UV Mapper can be used to add/remove groups easily.  You can do it in the Poser Setup room as well, but not sure how that goes for props.  Whatever modeling app you used exported with groups for some reason.  For instance, in Cinema 4D, separate objects can be treated as groups or objects with polygon selections have those selections treated as groups.  As a last resort, you can open the .obj file in a text editor and simply remove all of the lines starting with 'g'.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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