Sharing a song or a playlist allows you to create a link to send out. Sharing your song or playlist doesn't give the recipient any access to the song project, but it lets them see the song or playlist page and allows them to listen to and download depending on how you share it.
Adding members allows you to give other users access to song projects and playlists. You can add someone with Can Edit or Can View access.
Can Edit access is for when you want a user on Songspace to be able to make changes and contributions to a Song Project you're working on. Can Edit access is good when you're collaborating with another writer or artist.
Can View access is for when you want to allow someone to view and listen to a song, but you don't want them to be able to make any changes.
Note: You can also credit someone on a song project to help you keep track of who is involved on the project. Credited members do not have access and are not notified that they have been added to a project.