Just to clarify:
The passengers should NOT be boarded BEFORE the Flight or the CabinCrew! Especially on the first flight of the day.
Before anybody turns up there are a variety of security, safety and technical checks that have to be performed and all of those according to checklists. With all these checks and crosschecks forms have to be filled in by the various members, including the dispatcher, de-icer, fueller, caterer, engineers, cabin crew and pilots.
When the pilots approach "the cold aircraft" they check the general appearance: chocks, wings, ground power unit if appl, is it a cold, wet morning? Is there any ice on the ground? Anything else that could be slippery for the pax? A thorough walk round is performed shortly afterwards, and it should entail more than just kicking the tyres!
Then the cockpit gets a safety check: gear lever down, park brake on, all electrics off, emergency equipment in place: even the expiry date on the fire extinguisher is checked!
Then the plane is switched on: if you're lucky you have a little engine in the tail of the plane that will give us our own electricity and provide us with some air to run the heater or the cooler.
The technical first flight checks run through all the systems: electrics, hydraulics, pneumatics, all the warning systems, all the computers, all the paperwork, incl insurance certificates and radio licences, the tech log ( where you can read about what the engineers had to do overnight ) are checked, to name but a few.
Now: If there is a fault with anything that any of the crew in conjunction with their knowledge, training and experience is NOT happy to launch with, passenger boarding is delayed and the fault rectified in hopefully the shortest of time.
So: boarding passengers, without having the green light from the dispatcher, who has the green light from the Captain, is a big NONO!
The last thing anybody wants to have to do is deboard an airplane due to any fault that has been picked up during the first flight checks.
So, to answer your Q: The whole compliment of the crew should be there and briefed before you arrive.
happy landings
sparkle