The included images have assigned colors to each Smoothing Group to distinguish them and show where they are assigned to each model. The skins are 256 color .bmp files, each having their own palette. Parts of the skin are applied to specific faces of the model with the Edit Mesh Modifier. The UVW Mapping Modifier then adjusts the skin to line up correctly. They are done the same way as any other model, so here's a detailed Skin Texture tutorial to learn more about that.
Weapons actually do have a skeleton.
This is because the game engine only animates skeletons, not individual vertices, so a simple bone skeleton is created for weapons. The model is attached to the bones using Physique, a 3D Studio Max Modifier. Envelopes are not created and there is no blending between links. Bulge Angles and Tendons aren't used either. The Biped in the DMPlayer Model is the same Biped used for multiplayer models.