companion; fellow attendee; person sitting next to one
Refers to someone who shares the same seat, table, or attendance at an event. The specific nuance depends on context: a companion at a gathering, a fellow participant, or simply the person seated beside you.
At the meeting, I exchanged business cards with the person sitting next to me.
I enjoyed talking with my fellow attendees at the wedding reception.
Compound of 同席 (どうせき, 'same seat/attendance') + 者 (しゃ, 'person').