Troubles using "groups"

Finding it very difficult to use Groups. I created a group with 23 people. They are all outside my organization. Some of them are saying they are not able to view the files I had put in the group. I'd really like all of those to be available to everyone in the group. In addition, I log in today and it's not showing any of the files anymore. I don't know what happened. I feel like not having a good handle on this is making me look unprofessional. HELP!