Sure, here's how you can make your collaborative Spotify playlist private:

1. Open Spotify and navigate to the playlist you want to make private.
2. Click on the three dots (more options) next to the Play button.
3. A drop-down menu will appear, select "Playlist settings".
4. In the settings window, under "Collaborative Playlist", ensure the toggle is switched on. This allows others to add, delete, and reorder tracks.
5. Below that, you'll see "Make Secret". Toggle this on to make the playlist private. Only you and the people you invite will be able to see and play this playlist.
6. Close the settings window. Your playlist is now a private, collaborative playlist.

Remember, even though the playlist is private, any Spotify user with the link can access and edit the playlist. Be careful who you share the link with.