If the project has not been recorded yet:
Projects need to be recorded before they are in a music file format. Find the project you'd like to share and go to jam mode. Make sure the playback mode (in the top right-hand corner) is set to 'play all parts and stop'. When this is selected you should be able to record the project from start to finish by pushing the red button in the bottom right-hand corner.
When the recording is finished it will automatically ask if you want to share it, so you just need to follow the prompts for the 'Release' option which will post the song on your profile in the Community. It's also possible to go to My Music > Recordings, click on the project and select 'Share.'
If the project has already been recorded:
Go to My Music > Recordings, click on the recording and select 'Share' then choose 'Release'. This will post the song on your profile in the Community.