Hi, I’m on level 390 & a new player has just appeared the the scenes I’m playing but hasn’t joined..

Wooga starts everyone with a small set of players to play and exchange bushes, so they appear on the boat list and the scene leaderboard. Periodically that set gets updated with more active players. If you befriend anyone on Facebook, they will join your list, too. As you noted, your teammates are of course on that list.