Think of the beginning of a scene as an establishing shot in a movie. You must give the reader visual clues about where the scene is and who is in it. You must also tell them when the scene is if time has changes much from the scene before it.
If you have four characters in a car, you must tell the reader within the first paragraph or two that Tom, Dick, Jane, and Harriet are in that car. You can’t have Jane and Harriet chatting in the front seat for five pages then have Dick chime in on the subject in the back. That will jerk the reader right out of the scene as she tries to figure out where Dick came from.
In a scene with a lot of people, it’s not necessary to mention everyone who will be important, but you should show a character who hasn’t been mentioned as there come forward to join the conversation or whatever, and the viewpoint character should react to their presence. The new character may need to say why they are there if their appearance is totally unexpected.
Remember that you are building a visual and emotional scene for the reader, and you must offer enough information and clues to allow them to see the scene themselves, or you lose them as readers.