If you would like to temporarily block off your street for your party, you will need to work with the Portland Bureau of Transportation (PBOT) to obtain a permit.
Street closures of this type are classified as either "Block Parties" or "Community Events," depending on the size of the closure and the attendance at the party. There are slightly different requirements for block parties versus community events. Insurance is optional for block parties, but required for community events. Permit fees are waived for block parties registered for National Night Out, but still required for community events. Either way, you will need to rent barricades from a barricade company, which usually costs about $50 for a basic street closure.
The details are explained on the Community Event and Block Party Street Closure Permits webpage on the PBOT site, and you can call their block party line at 503-823-4003.