Hi Alex,The Administrator of the Organization can allow for content to be shared publicly or only within the Organization. There is a setting that they can change to enable this for you. http://resources.arcgis.com/en/help/arcgisonline/010q/010q000000mq000000.htm
If its policy to not allow everyone to share content to the public, then they could promote your account to Administrator since Admin's can always share publicly.Hope that helps,Andrew