This is something we talk about often in rehearsals.... who is important right now? The answer is obviously "everyone", however, in group scenes it can be a bit tricky.
Let me explain. Everyone onstage is visible, therefore they need to always be present, to be actively engaged. They need to know how their character would react to each line of dialogue that is being said, as if it's for the first time. They might agree, or be shocked, or be angry etc. They might be amused and laugh along (with the audience perhaps). Of course, sometimes they may not "hear" the dialogue at all, then they will need to stay occupied elsewhere.
The question for all actors therefore is, how big should my reaction be? Secondary characters in a scene are so important, but they need to find the sweet spot - staying present (in character) yet not upstaging the action. Upstaging is when a secondary character creates a diversion to the central action and takes attention away from the actual story. A good director will reign in their actors if this happens, to ensure the audience stays focussed on what they need to.
Sometimes with inexperienced actors, the opposite happens - their attention onstage can wander. This is very obvious to the audience as they don't react appropriately and can also draw the audience attentions for the wrong reasons.
This is a photograph from our recent production and is an example of excellent teamwork and engagement. Audrey (L) is a proud lawyer with no morals, Marsha (C) is Lady Macbeth who is thrilled with the plan, and Lauren (R) is a nurse who is actively listening. They each have different reactions suitable for their characters at that time.
Remember, theatre is teamwork and everyone (cast and crew) need to pull together to tell the story.