There are many discussions in this forum that deal with GAL functionality, configuration etc. Do a search and see if you can find out what you are looking for.
Now, personally I find clicking the "TO" button on then adding people one by one to be slow and tedious.
Instead I find it far more efficient to just start typing a persons name and letting the GAL auto-populate my TO field as I go along.
To ensure the GAL auto populates as a user types check your COS settings under "Features" and make sure "Auto-complete from GAL" is selected.
The webclient should also include the users personal contacts folders when auto-completing.
"Let's look at this from a standpoint of "Status". What exactly, on the Space Craft, IS working?"
-Flight Control, Apollo 13