To add songs to a collaborative Spotify playlist, you need to follow these steps:

1. You must have a Spotify account. If you don't have one, you can create it for free.

2. You need to be following the collaborative playlist. If you're not already following it, you can search for the playlist and click on the 'Follow' button.

3. The owner of the playlist must have set the playlist as 'collaborative'. If the playlist isn't collaborative, you won't be able to add songs to it. Only the owner of the playlist can change this setting.

4. Once you're following the collaborative playlist, you can add songs to it. To do this, find a song you want to add, click on the '...' icon next to the song, and select 'Add to Playlist'. Then select the collaborative playlist you want to add the song to.

Remember, changes made to a collaborative playlist are visible to all followers. If you remove a song, it's removed for everyone. If you add a song, everyone can see it.