When there's a scene with a group of characters together on a mission or journey, and the most dynamic 2 or 3 of them are in a conversation or working on something, what do you do with the other people? It's not realistic for them to just stand around doing nothing, so there probably needs to be some mention of them, right? You can give them a task, like "..and Josh and Megan headed off to test the equipment" to get them out of the way and be doing something plot-relevant. What else? I realize this is super vague. That's because I want to focus on strategies for this type of thing in general, not a solution for just one specific situation.
My feeling is that, if they're not mentioned -- unless this scenario is clearly something they'd be directly involved with -- the reader probably isn't going to be going, "Gosh, I wonder where X and Y are in all this?". They're probably going to be focused on what's actually being shown to them, not preoccupied with the whereabouts of characters who aren't currently relevant.
First off, you might want to look at the plot as a whole and make sure characters can't be completely cut or merged together. Second, you can examine the specific scene in question and try changing it to make the less involved characters have a more active role. Third, you can--as you suggested--get them "off screen" on some task, then bring them back in when appropriate. As long as there's a good reason, this works consistently. Lastly, having them standing around doing nothing could be realistic in the proper context. Maybe they're stuck waiting for someone to open a door from the other side or whatever. It's hard to say whether it would work without a specific example to look at. I'm sure there are other possibilities, but those are the ones I can come up with off the top of my head.
In first person, the character can mention other characters. In third person the narrator can mention other characters, just like you mention anything else in a scene. Where they went, what are they doing, maybe they can have something to say. There's not much else you can do. Have you read a book about groups of characters like that? That could give you some ideas. If you watch "Fellowship of the Ring", you'll notice that each character does something. If Aragorn sits by the fire while Frodo and Legolas have an important talk, then you are still mentioning Aragorn, you're not ignoring him just because he's not doing anything very exciting. It's perfectly okay to have a few paragraphs about some of them and leave the rest out, but if you have some characters do nothing for most of the book then the reader would start wondering why you bothered to put them in in the first place.
I second this. I think, that if the "unused" characters will only be unused for a small amount of time, that it probably isn't even really necessary to say anything about them, unless of course, you want to. The only exception I think is if some characters will be gone for a prolonged period of time, you should probably mention why they aren't there.
Depending on your scene, if only a couple of people in the group are interacting in a way that's important to the story, I wouldn't worry about placing each of the other group members and giving each of them something to do at the same time. Think about how you would read a scene like this, or maybe find a few group scenes in books you like, and see how other authors handle this kind of thing. If the conversation or action between your two or three main characters is happening in public, then you could give insight into how their actions or statements is hitting the other group members. (They might be agreeing, or squirming uncomfortably, etc.) However, if the interaction is private, there really isn't a need to place everybody else in the group. A quick mention of what the group is doing as a whole will probably suffice. Everybody else is dancing, sleeping, walking around, getting ready for something, talking and laughing, hacking the enemy to pieces, and etc. Unless some particular dynamic is in play (half of the group are getting ready to fight, while the other half are getting ready to run—and one or two important people have already left) the reader will just assume it's business as usual with the other group members.
Concur, had that situation come up many times in mine. No need to give them some meaningless task, which will probably annoy the reader, because you are constantly listing the whole cast even when they are not all involved. Just ignore them when inactive.
