Regulus Black is a neurotic Unspeakable who doesn't really get out much, it is so tragic. He is a Quidditch fan, so he would know of Sean, though I doubt they've ever met in person.
Peter Pettigrew owns a security firm and private detective agency in Knockturn Alley, and he probably wouldn't know Sean unless they had professional dealings at some point.
Ken Chambers was a Ravenclaw, class of '96, and a Chaser for their Quidditch team. He is now a rock star, the lead singer and guitarist for the band Quake. Most people know who Ken is, and he definitely knows of Sean because he loves Quidditch, but they probably wouldn't be acquainted? BUT THEY SHOULD BE.
Parvati Patil was a Gryffindor, class of '98, and is now a Daily Prophet reporter. She can come across as a bit spacey or silly, especially if you don't believe in Divination, but she's smart and knows a lot about current events from work. She's always up for making friends.
Gabrielle Delacour is precious and sweet and new to England, and she would totes be friends with Sean too!
Finally, David Shafiq is a bit of an odd duck. His father was a Death Eater who kidnapped him from his mother when he was a baby and raised him in total isolation. David was only introduced to wizarding society in December, when his father died and the DMLE found him. He's very inquisitive and not entirely aware of social norms for obvious reasons, and he'll talk to anyone if they pique his interest.