By joining a community, you can see updates being shared by other users in that community on your Profile wall.
In order to post in the forum for a particular community, you need to have joined that community.
You can join one or many communities and can leave a community at any time.
In order to join or manage your communities, visit the Communities page.
You can search for a community to join by using the search box or by browsing the categories or alphabetically by condition (A to Z).
Click on the name of the community you wish to join, this will take you to the Forum for that community.
Select Join Community. You are now a member of this community.