Although "estar" + "participio" reminds the structure of the passive voice, it isn't. And in this case it doesn't really make sense.
The construction "estar" + "participio" is called a "perífrasis de participio". It is formed by the verb "estar" with "participios" that refer to the resulting state of an action, or something that became transformed at the end of a process. The passive voice talks about someone doing something with a purpose.
- Estoy abrumado por los problemas. -> What you say when a normal state of peacefulness has been altered by the appearance of problems.
- Soy abrumado por los problemas. -> What you'd say if problems had an active will to overwhelm you.
- Estos juguetes están hechos por artesanos mexicanos. -> These toys are here thanks to the work of Mexican artisans.
- Estos juguetes son hechos por artesanos mexicanos. -> The Mexican artisans are making these toys.
- El tráfico está detenido por una manifestación. -> The cause of the cars being stopped is that there is a demonstration.
- El tráfico es detenido por una manifestación. -> The people in the demonstration are keeping the cars from moving along.
- Las puertas están cerradas. -> That's the state of the doors after having been put into this position.
- Las puertas son cerradas. -> Someone is closing the doors.
Edit: If you say "estoy invitado a una fiesta", it means you have this status as possible guest to the party that hasn't happened yet. Still not the same as "soy invitado a una fiesta", which would mean that you are being invited to a party by someone.