Well, can they?
No – if you don’t believe in ghosts
No – if you accept, as the definition of seance: “a meeting at which people attempt to make contact with the dead,” and you don’t think ghosts are people.
Yes – if you define seance as “communication with spirits,” and “with” includes the concept of “among.” That would cover the times ghosts get together to shoot the shit, dish gossip, etc.
But a bunch of ghosts trying to contact the living? That would be an ecnaes, not a seance.