Sharing modules among DNN portals (i.e. site groups) is not an out-of-the-box functionality in the community edition; however, it is not completely unavailable. Starting from DNN 6.1, the functionality is built in the core, so no matter what edition you use, it does support module sharing. It is just "hidden" in the community edition.
Here is a simple way to "unhide" the functionality in the community edition. I tested it on DNN 7.0.0 - 7.1.2, and found out that ... read more