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. Both homeowners and renters may request a street closure permit and throw a block party.
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. Block parties can be organized year-round, with the majority of parties occurring during summer months; registering a party as a National Night Out party provides additional benefits, which can be found here. There are slightly different requirements for block parties versus community events. Insurance is optional for block parties, but required for community events.
As of March 7, 2018 permit fees are waived for all block parties (including block parties taking place during the week of National Night Out), but are 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; however, in certain areas some neighborhood organizations have a limited number of barricades that are no cost to borrow; more information about that can be found here.
The details for block parties are explained on the Block Party Street Closure Permits webpage on the PBOT site, and you can call their block party line at 503-823-4003.