In the beginning, there was the Committee. And the Committee decreed that there shall be a Door Group. Or something like that.
Door members are simply normal people who are trusted with the responsibility to unlock the clubroom and to allow other people to use the room in their presence. Door members also get access to the tool cupboard, machine room and vending machines via the keys in the lockbox. Naturally, door members are the face of the club and they are expected to behave in a friendly and welcoming manner.
Any ordinary club member can apply to be added to the Door Group ("apply for door"). This is fairly simple and simply involves sending an email to the Committee with an explanation of why you want to be on door and a summary of your understanding of the Club Regulations (mostly just the Door Group bits).
To contact the Door group, please send an email to the Door Group mailing list. Note that this is also the official point of contact with the committee for door members.
Door Group Manual
The manual is a brief guide to many of the technical aspects of being a door member. It's a good idea to familiarise yourself with it if you are intending to apply for door.
See the manual itself here.