You must be a registered user on the site to contribute any materials. Only members of a particular group may post to that group.
The easiest way to upload documents and materials within a private group (one that you are a member of),is to:
Login to the site,
Find your group by clicking on the "Groups" link at the left of the page, then click "My Groups" and select the group you want to upload materials into.
Select the "Create Content" link at the left of the page
Select the kind of resource you want to use (Page, Story, Forum Topic, etc.) and fill out the required information.
In the "Audience" section, check the box to connect the posting to a particular group. Once checked, it will only be visible to members of that group.
To upload a file, click on the "File Attachments" link, browse for the resource and click on the "Attach" button.
When finished, don't forget to save!