You can only communicate with people that are in your
group.
In the Community dialog box, you only see the content of
your current group. You don't see the content of any of the other groups.
(This is logical, since you can only communicate with people in your
group.)
Before examining group content, it is recommended to do a
Refresh of your Community dialog box by right-clicking the Community
root and selecting Refresh from the contextual menu (since
when a person changes group, his / her Community db is
automatically refreshed, but the other people who are impacted by the
change will not have their Community dialog boxes automatically
refreshed).