Just a few quick things that I learned while trying to create a new context menu item on the solution explorer in visual studio:

First, there is a post on how to enable VSIP Logging that comes in handy to figure out the Guids and Ids of the menu items in visual studio. The full post is: Using EnableVSIPLogging to identify menus and commands with VS 2005 + SP1, but this part applies to Visual Studio 2010 as well. Essentially you flip a flag in the registry, restart Visual Studio and then using Ctrl-Shift, Right Click on menu items and a dialog appears with the information that you need. Here is the registry key:
[HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\8.0\General]
"EnableVSIPLogging"=dword:00000001

Second, from an article entitled Dynamic Menu Command in Visual Studio Packages – Part 2, which is where I found the first reference from, it shows  how to define a GuidSymbol, Group and Button appropriately. I am not going to repeat that here. Really it is just repeating what Visual Studio does for you automatically when you create a Package.