In recent days, so far as screen drama portrays life there is a need for all types of people: characters and neutral anonymous faces, old and young, smart and scruffy. In fact, everyone you might ever meet has a place as an extra.
If you want to be extras, you do need to be able to drop everything and take work when it is offered (this is not usually a full time occupation). You must get yourself to a studio or location with less than 12 hours to plan (locations can be remote from public transport) and be ready to get up at 6am or earlier to make whatever time you are asked to be on set.
Most people who want to be extras have such questions “Do we need to have training classes”. But you must be ready to follow instructions and unselfconsciously get on with whatever is required in as natural a manner as possible. It is obvious to say that you never look at the camera or attract attention to yourself without direction. The most difficult thing you will possibly be asked to do is to mime a conversation, ie, pretend you are talking to someone without making any noise, so that the principal actors will be heard.