Learn English – The word for people stacking up on top of each other

I don't know a single word that would replace "a group of noisy people". There's a whole bunch of adjectives that could describe that group of people - amongst synonyms of noisy (bolding ones that would match the situations best),:

chattering, disorderly, ear-popping, ear-splitting, loudmouth, rackety, raising Cain, raising the roof, rambunctious, riotous, rowdy, screaming, tumultous/tumultuous, turned up, uproarious

and quite a few nouns to describe the situation - synonyms of ruckus

disturbance, big scene, bother, brawl, commotion, disorder, disruption, fray, fuss, hubbub, racket, stir, turmoil, upheaval, uproar

Actually, I think good-old "free" is the best choice, as in:

having no obligations (as to work) or commitments;

not taken up with commitments or obligations;

not obstructed, restricted, or impeded;

but especially:

not being used or occupied.

I'm not sure what you mean by "They are about to help others in other activities", but I presume you mean that, in a work situation, they have finished their task but are still at work (and expected to be working). In this case, you would not call this "free time", as they are not completely at their leisure. "free time" is usually reserved for non-restriction, such as outside of work, or during work when there is absolutely no work to do and no one to help.

Referring to the person as "free" is still appropriate, mostly in the sense of the last definition: they are "not being used or occupied". It is common to ask someone in the workplace "Are you free?" meaning are you not busy so that you can help/do something?

"idle" would suggest that the person is not doing anything useful. Assuming they are about to help others, they are not really idle, but they are free to help others.

"available" is also perfectly appropriate; "are you available?" is in fact a more formal way to ask "Are you free?"