Rasterize and Merge All Layers Inside Group(s) in Photoshop

There is one very handy command and shortcut to rasterize and merge all layers inside layer group(s) in Photoshop. This s in brackets means that you can select multiple groups of layers to rasterize every layer inside all groups and to merge everything to one single layer. Command Merge Group is available on right click (Cmd + click) on group of layers or from Layer > Merge Group when group of layers is selected.

By the way you can use Ctrl/Cmd + E keyboard shortcut to achieve same goal. Merged layer will pick name of group or top most group when you use this command with multiple groups. To rasterize and merge content from multiple layer groups first select all groups then use same shortcut or command as for single group.

Merge multiple groups using Ctrl/Cmd + E keyboard shortcut

Merge layers in group(s) non-destructively (Stamp layers)
If you want only to create merged layer with content of group(s) without to rasterize and merge layers, in fact layers will stay intact as they are, first select group or groups of layers then press Ctrl/Cmd + Alt/Opt + E keyboard shortcut. See also Merge Layers Non-destructively in Photoshop.

Stamp layers in Adobe Photoshop

Reference:
Photoshop Help / Managing layers

Product used in this tutorial: Adobe Photoshop CS6