I am attending an event in Reus, outside of Barcelona. I will have a car, but would like to go into Barcelona city center without the car. Is there a Metro stop I can park at, then take the train into city center?
Reus is not "outside" Barcelona by a long shot... it's over 100km from Barcelona, or way over 1h drive.
Anyhow, I'd simply take the train in Reus... and get off in Passeig de Gràcia station in Barcelona. The ride will take slightly over an hour and a half. Note that driving to Barcelona wouldn't save you much: entries to the city are quite busy most of the time, and you don't know your way around so you might need some time to get your bearings -even with a TomTom! You'll save headaches, motorway tolls and (expensive) parking cost in Barcelona -yes, there's no free parking here, not even in the street.
If your time is tight, you could/should stay in the area. Reus in itself is worth visiting, there are several interesting sites among which the Gaudí Museum (he was born in Reus!). But also, you have Tarragona at a short hop from there. Tarragona was the former capital of the most Western province of the Roman empire and has several sites displaying its former glory, including a fully-fledged Roman circus.