By "background", I mean the dialects, habits and culture of the community. Also, I wonder if it is even possible to write about a community without being a part of it?
I am only asking this in terms of writing fiction.
on Writing Excuses, they recommended that in this situation you take one thing and explain the heck out of it, then take another and don't explain it at all.
For cultures, you can put in one or two things from your study of the culture, and really go into detail to the point where it looks like you know what you're taking about. Then you won't have to go into as much detail for other aspects, but you'll still (hopefully) have a very authentic feel. The result is that it looks like you could have described everything in more detail, but, like any good fiction author, you're focusing on the action/characters/story.
In a fight scene set in ancient Japan, study up on some etiquette and weapons, describe those in detail ("Yojimbo drew his sword and dropped into a fighting stance, waiting mutely for his opponent to acknowledge the challenge by doing the same" and "the sword was his grandfather's katana, forged from fine steel folded a hundred times for tensile strength"), and then leave out explanations for the other details.
(Note: I haven't actually studied ancient Japanese etiquette and weapons for these examples, but hopefully I've illustrated the point).
It's pretty important (if a significant portion of your audience knows as much or more about the subject than you do) to have an alpha reader who is an expert in the subject matter proof-read it when it's "done" and correct any glaring problems.
Be very careful with dialect. It's difficult to do well (if it's even possible). It's easy to offend. And, most importantly in my mind, it's very annoying to read. Word choice is going to show your character just fine -- if you want ignorant or affected, rich or poor -- as opposed to actual dialect.
It depends on your intent; if your work be it fictional or factual, hangs on details about this community, its customs and practices,then you need to research scrupulously. If it's broad strokes, background characterisation, or local colour, then a more general level of knowledge is acceptable. In either case, you cannot give the same level of insight as a member of that community but you should be truthful, avoid lazy stereotypes, and if your research shows that your initial ideas about that community or culture was inaccurate you should rethink your approach. Also, while you cannot describe it as an "insider" you can give an objective point of view that is just as valuable and interesting if you try.
It depends on your own goals and priorities.
Maybe it is even impossible to write about community without being part of it?
Think about your own community. Imagine someone from a completely foreign culture reading up on it for a few weeks and then attempting to write a novel based on it. No matter how hard they studied, they would inevitably get things wrong -- some of them being plain old mistakes, and others being subtle things that can't really be explained but have to be experienced to understand why they're wrong.
But it might not matter. Stephanie Meyer knows sweet fuck-all about Native Americans, but that didn't stop her from exploiting them ruthlessly in her novels and she's doing alright.
Depends on what you are writing I guess. If you are writing an article on that specific subject, I would dive pretty deep into the matter. You usually get to a point where you feel confident about what you are saying is right. Don't throw out assumptions. If someone later came and told you "that's wrong", and you have no way of defending it, you obviously haven't dug deep enough.
If on the other hand you just need it to emphasize a character or a specific environment in a bigger story, I would talk to someone who really knows the field and get them to come up with a few typical examples of behavior/descriptions/word that only someone really into it would know, that at the same time comes as a natural extension of some preconception that most people have about the subject/area/culture/thing.
If you write a documentation about this community you should naturally investigate deeply into the topic. But I think you mean using characters or elements from this culture in your fictional story. In that case you must see, that most of your readers possibly don't know more about the community than you does. So you don't need too much investigation, in the end it should suffice, that you know a little more than most of your readers. Members of this community on the other hand, will always know more than you, even if you investigate strongly. It happens all the time in novels or movies, that a community is shown partly wrong. It doesn't matter, if the main story is good enough.
But one thing you should consider: you may don't want to offend members of the community. So you might learn about offending stereotypes you should avoid.
You should investigate to the point that you have a good grasp of what's important to the community members and what's not important, especially on the topic of how that community is depicted.
The Golden Rule applies here. Would you want someone writing about you to leave out details that put what you've done in a different light?