You may be able to find them if they listed any games other than D&D5E by searching those games in the player directory, if you know what handle they signed in as. I often find people have multiple names. An email name of Scarlet avenger, on Reddit might be batman355 and be Little Pony 666 on roll 20. D&D5E has about 2 million players, so it could take a lifetime to find one. Also if you have contact outside roll 20, create a game and invite them, even if it is just a place to plan games.