First, you want to select the parts of your func_door, hit n, and give them a key called "team" and a value of "group_1" (Giving the door the team key makes all of the parts of your func_door move together at the same time, if you only have a door with one part, then don't add this) Next, give them another key called "targetname" and a value of "group_1" Now what you want to do is make a trigger_multiple surrounding the area you want the door button to be. Give the trigger_multiple a key called "target" and a value of "group_1" Then check the "use_button" spawnflag box. This will make it so the door can only be opened if someone is in this trigger and pushes their use button. Good luck.
~Mike "Zer0-L0giC" Majernik