Learn English – A word for good part of something

I would suggest utilise

I instinctively spell this ending in -ise, but utilize is the common spelling internationally. I found this useful entry which elaborates further;

Utilise or Utilize

For example, "He utilised his experience in order to solve the problem".

From Collins

to make practical or worthwhile use of

The problem is that, much of the time, such behavior is seen as a sign of introversion, so the available expressions like wallflower denote shyness rather than something more productive like working contentedly in a busy coffee shop.

Meanwhile, a word like bystander emphasizes nonengagement, but in no way addresses the state of the person doing it. Are they content? Scared? Furthermore, a bystander suggests an activity or event: it doesn't make much sense to be a bystander in your own home when friends are over. As for its synonyms, onlooker and other options suggest a kind of observation that need not be present. You're not necessarily looking or listening all the time; you're playing video games or watching Netflix.

So because the direct options are imperfect, I suggest going more general and then specifying what you mean: you companion or accompany (verb) or you are companionate (adj). The verb usage of the word companion is fairly unusual, but it is attested in dictionaries like Merriam-Webster:

transitive verb

: accompany

He is companioned on the trip by his wife.

intransitive verb :

to keep company

fellows that he'd companioned with many years ago

Keeping company can entail a broad range of activity, from direct interaction to having people in the same space as you. Hence you are companionate: you suitably or harmoniously accompany others. You would need to specify how these terms should be understood through context, but in the context of your sentence with its gloss the phrasing would make sense:

She was truly companionate; always happy with people around but never feeling the need to join the conversation."

Or, with the verb and some modification:

She accompanies rather than participates; she is happy with people around but doesn't feel the need to join the conversation