I believe it’s important for all writers, at some point, to make themselves members of a writers’ community of some sort. I don’t think this necessarily needs to be a community of other writers so much as it should be a community of people who either knowingly or not assist the writer in some way. This can mean they are readers who happen to point out issues in the writers’ works. It is by no means necessarily true that a writer’s best hopes for success lies in them becoming a member of a writers’ community which consists only of other writers; sometimes, such communities can actually hold back a writer, particularly if the person in question is advancing in some way and the rest of the members have and continue to remain pretty static in their writing.
So, as I see it*, a good writers’ community (particularly one consisting only of other writers) will:
1. Help the writer improve their writing skill. Whether this means they assist in the teaching of technical skills (spelling, grammar, etc) or in creative skills (how to worldbuild for fantasy/sf, how to insert research into a story in a creative way which isn’t overwhelming, awkward, or unnecessary), or just in giving the writer a few like-minded individuals who provide thoughtful and thought-provoking critiques which aid in the revision process, I think a good writers’ community will aid in this aspect of writing as much as possible, particularly for beginning writers. Some groups will be better at these things than others, so, if possible, a writer, especially one who needs help with these things, should do their best to find a community which can help them learn these skills.
2. Provide a supportive community. It’s important for a writer of any experience or skill level to feel supported and encouraged by their writing community. Whether it is online or in-person, if a writer feels ignored, unsupported, unencouraged, or in any other way uncomfortable with the group, they shouldn’t stick around. A group where a writer feels unwelcome or uncomfortable is in no way any kind of help. If there are a few who a writer gets along with and it’s possible, the writer should make an effort to stay in contact with those they feel to be the most helpful, welcoming, and supportive.
3. Provide a network to assist in attaining goals. This goes in some ways along with being a supportive community, but has some aspects all its own. A writers’ community which doesn’t appear to encourage goal-setting isn’t a good fit for someone who wants to do so. There are some groups whose members are writing purely for the fun of it, and they also may not care too much about helping to improve a writer’s skills or be supportive in other ways.
4. Have goals which mesh or at least somewhat align to the writer’s goals. In addition to assisting in attaining goals, the goals of other group members should in some way lean toward focusing on the same goals a writer has. For example, if a writer has a firm desire to be traditionally published (get an agent, sell rights, etc), the group should be prepared to help attain this goal. This might include, for those wishing to be traditionally published, help in writing query letters and synopses, advice on contracts and when and under what circumstances to sign them, and factual information on the path to becoming traditionally published.
5. Relates information regarding writing and whatever goals a writer has in a reasonable, informative, respectful manner without insulting the writer’s intelligence, regardless of what level of skill, experience, and intelligence the writer brings to the group.
6. Should not necessarily be focused on one type of writing, but, if it welcomes writers who specialize in different types of writing (for example, scriptwriting and fiction), should have a structure which enables whatever writers are members to relate to other writers of their type of story. This should not necessarily be exclusive, as we all can learn from one another, but it should be made clear to everyone that what works for one thing (say, scriptwriting) may not necessarily be the best advice to give/follow for someone who writes prose (or nonfiction, or poetry, or whatever) and vice-versa. And when such advice is given, there should be someone who is knowledgeable enough in those types to be able to advise on whether that particular bit of advice works for the type given.
These are things I’ve picked up over the course of my writing life. I’ve been a member of a successful, supportive online writers’ community for over ten years, and I have attempted to join a number of in-person writers’ groups, but could not stay consistent with them for various reasons, some because I lacked the transportation I needed to them, and one or two which were not a good fit for me for whatever reason. Of those which were the best, they all provided at least three of the above things for me. I imagine, however, I would have in some way outgrown all the groups that worked for me at some point. The one I have yet to outgrow is the online group; it is the most comprehensive writers’ group I’ve ever been a member of, and it has retained its flexibility, openness, and supportive aspects through changes of hands and moving the site.
*My opinion only; your mileage may vary a great deal depending on what is specifically being sought.