Users mailing list: httpclient-users@ for general discussion, questions, and announcements. This is the perfect place to ask for help if you need it!
Development mailing list: dev@ for discussion about project development.
GitHub: Star us at apache/httpcomponents-core and apache/httpcomponents-client and use this to follow HttpComponents development or contribute pull requests. If you're interested in development, please see the Contributing section below for details on our development process.
Slack: Some committers and users are present in the channel #httpcomponents-client on the Apache Slack team.
StackOverflow: While the user mailing list is the primary resource for asking questions, if you prefer StackOverflow, make sure to tag your question with apache-httpcomponents.
HttpComponents is a community-led project, and we are delighted to receive contributions of anything from minor fixes to new features. If you have an itch to scratch, then by all means do that! Fixing bugs you run into, or adding features you need, are both immensely helpful. There are plenty of ways to help outside writing code. Code review of pull requests (even if you are not a committer), feature suggestions, reporting bugs, documentation and usability feedback all matter immensely.
As it happens, the ways to get involved into an open source project are basically the same for all projects. Please have a look at the Incubator Guide To Participation. It's terse, but has pointers to additional information elsewhere at Apache. The sections on participating as a User and as a Developer apply here, too.