Can't speak for Lightroom but in Photoshop (I am using CS5) the first thing you need to do load all the different pictures into layers of a new image. There is a tool in Bridge to do this Tools>Photoshop>Load Files into...
If all shots are taken from the same angle and with the same zoom it will be straight forward but you may need to align the images if there is a common background. There is a tool for this in Photoshop, select all the Layers then go Edit>Auto-Align Layers, but this can be hit and miss. If this fails then you will need to use the Transform Tool on each layer individually, lowering the opacity so you can see what you are doing.
As for merging the images there are a couple of automatic things to try depending on what you want.
First, try selecting all the layers and going to Edit>Auto-Blend Layers. If that's not what you want then...
Second, select all the layers then go to Layer>Smart Objects>Convert to Smart Object. Then go back to Layer>Smart Objects>Stack Mode>Minimum. If that doesn't do what you want then try other stack modes.
If all the automatic methods fail then you will have to do in the old fashioned way! Give each layer a layer mask then use the Brush Tool to paint in Black/White the bits in each layer you want to Hide/Keep.
There may be some super auto plugin I am unaware! But good luck