The organic groups module allows you to create a working group where a number of people with shared interests can create content which is non-public, visible to group members only.
To be able to create and moderate a group, you need to get 'group moderator' permissions. To get this you need to ask the Administrator.
To create a new group you first need to create a new content type.
In the navigation bar under your name on the left side bar, which I call the admin menu, go to 'content type', then select 'add content type' choose a name and a type (same as name but machine readable), tick 'published' but not 'promoted to front page'.
Under 'organic groups usage' tick 'Group Node', this is mandatory, everything else select to your liking and save content type.
Then go to 'create content', you will find your new content type there, select it and everything else is pretty self explanatory, by creating the first content node of this type you set up the group. Select
'private' maybe and 'moderated', do not offer it for 'selection on registration'
You can decide if your group should be listed on the 'groups' homepage. I would suggest to say 'yes' to that.
Now, to create content for your group, click on groups, and then on your group. Once you are 'inside' your group, there is a new 'groups' menue on the lest sidebar; you will probably have to scroll down a little bit to get there. Select one of the content types offered there.
You can then decide with each article if you would like to publish it only in your group or visible to all.
Here you go. Enjoy group life;)
note of caution: as a group moderator you are given quite a few rights. please use those rights with caution.
There is some more information here:
http://drupal.org/handbook/modules/og