Fusioncept has "playlists already made and want to add [a] song" -- you can do this.
Coffee wants to "go into my music, select tracks, and add them to a new or existing playlist." -- you can do this.
The real smackbo might be "jammin to a song and want to add it to your current playlist" -- you can do this.
All three of you can do what you want, but you may need to think of the process differently. The instructions below are an extended version of my two earlier replies.
In Windows Phone, you edit Now Playing, not the playlist objects themselves. So, to do what you each seek, start playing the playlist -- this copies the track names from the playlist to Now Playing.
Once the music is flowing, go in to your music, put your finger down on the text of the name of an genre, artist, album, or track until a popup menu appears, then tap "Add to Now Playing." Repeat as needed for all of the things that you want to add to the playlist. Do not just tap the item or its art/play button that's shown next to the text; doing so will empty the Now Playing list (which currently contains a copy of your playlist) and then fill Now Playing with the names of the tracks from the genre, artist, album, or track that you just tapped.
When you've added everything, you have an expanded Now Playing playlist. You can save this; you can even save it over an existing saved playlist. To do so, go to the Now Playing list (the easy way is to tap the volume control then tap the lower part of the volume bar where the tiny print lists the next songs to be played). Once there, tap the "..." at the lower right and choose Save To Playlist. You'll be prompted for a name to give the list; type a new or existing playlist name. (For safety, if you type an existing name, you'll be warned before the phone overwriting the existing playlist.)