You can only share files if you uploaded the file or you are an admin. This can only be done from the website.
- Go to Collaborate
- RIGHT click on the file or folder you want to share
- Click Share
- Type in the group you want to share with or click view roster
- Make sure the proper groups are listed
- Click Done
Now your members will either see them in the Shared with me link on the website or the Documents link on the app.