how to add groups of apps on to dock ?

While holding down the Command key, select the apps in the Finder that you want in the Dock.

Then, just drag all of them from the Finder into the Dock.

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If you want to have a group of apps in one specific spot on the Dock, you have to do a few things.

1. Create a new folder someplace in the Finder (command-N). Name it something useful.

2. Select all of the apps in a Finder window that you want in that one specific spot in your Dock.

3. While holding down the Option AND Command keys, drag those apps into this new folder.
-- The apps will not actually be placed in the folder, but an "Alias" of each one will be.
-- Aliases are tiny files that "point" to the real files.
-- Aliases have a tiny black arrow in the lower left corner of their icon.

4. Drag this folder into the right end of the Dock.
-- An alias of it will be placed in the Dock; the Dock only has aliases in it, by the way.
-- Now, whenever you select that folder in the Dock, your group of apps will appear for you to choose between.

On the right end of the Dock is the (immovable) Trash Can icon, which is an alias for a "hidden folder" called ".Trash" which is found in your user folder, or "home folder".

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In the Dock, to the left of that Trash Can icon is an area where you can place files and folders, and to the left of that area is a faint white dotted line, and to the left of that line is the rest of the Dock where only individual apps can be placed.

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When I wrote "Drag this folder into the right end of the Dock" I was referring to the area between the Trash Can icon and the dotted line, on the right side of the Dock, near the right end. I apologize for not being more specific. Drag any file or folder to that location on the Dock and nearby Dock items will move apart, creating a spot for you to drop your file or folder.

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Here is the official Apple support document on what you are asking about, called "Stacks"... http://support.apple.com/kb/ht3739

http://www.macosxtips.co.uk/index_files/stacks-keep-dock-tidy.html

Please re explain in more detail.

Drag&drop!