Be on the door
Definition 1 of 2
Pro Tip 1/3Event Access
The phrase means controlling who enters an event, like a bouncer or ticket collector. 
If you're early, wait by the entrance, my friend will be on the door.
Pro Tip 2/3Temporary Role
The position of being 'on the door' is usually not permanent but for the event's duration. 
Tony is only helping to be on the door for tonight's fundraiser.
Pro Tip 3/3First Impression
The person on the door often gives the first impression of the event to attendees. 
Be friendly since you'll be on the door greeting our guests.