I’d add a new release corresponding to the YouTube playlist within the existing release group. I think that https://www.youtube.com/playlist?list=PLiJsK_sFeeKygEepqlUoXIBwcyeGfXrXx is the correct URL to use in a “stream for free” relationship. The existing recordings should be used when appropriate (i.e. when the songs from both sources sound the same and are the same duration).
If songs are still being added to the playlist, I don’t think that there’s clear official guidance about how that should be handled. There was some discussion last year:
Personally, I’d probably just update the release corresponding to the YT playlist to reflect the new tracks and consider documenting the additions in the annotation. Other editors prefer adding a new longer release whenever tracks get added, but that feels like it could get messy if new tracks are added every month.