In order to allow members from the same community to communicate with each other about several subjects simultaneously, you can use the Forums module. You can create as many forums as you deem necessary, according to the needs and requests of the members of your community.
You benefit from an archiving system and a moderating system, the latter allowing you to control the contents of messages left by your members. You also have the option of sending copies of these messages by email, which makes exchange possible even when disconnected.
Finally, the forums and their contents are protected and have secured access.