When you say join 2 polys, are you referring to simply merging two touching polygons together or joining them with extra geometry?
For the first, simply select your two polygons and then apply the Detail/Merge Polys command ('Z')
If the latter, then you have a couple of methods. If there's space between the two sets of geometry (ie: two faces with nothing in between, no shared points, etc), then you need to create extra polygons. Select the points you need to join in a clockwise direction and press 'p' to create a polygon from those points.
If the two polys have points that are nearby, and simply need to be joined together, then you can simply select pairs of points and apply the weld ('^w') tool.
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