Change the default address book
Open a new email message and click on the "To" button
Your address book probably defaults to "All Users - ..."
You will need to hit the down arrow to the right of that and choose "Global Address Book" and then find the person you are looking for OR you can make "Global Address Book" the default by doing the following:
Click on the "Address Book" option on the far right of the ribbon
Click "Tools", then "Options"
Select "Start with Global Address List" if it is not already selected.
Click on the down arrow to the right of "Choose Automatically" and then select "Global Address List".
Click OK to close that dialog box.
Hit the X in the upper left to close the open dialog box.
You're done.
Now, when you start a new message and click the "To" button, the default Address Book will be the "Global Address List".
Keep Meeting Requests even after you've responded
In Outlook, click "File", then "Options"
Click "Mail"
Scroll down until you see the "Delete meeting requests..." item. It is checked by default.
Uncheck it
Click OK
You're done.