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.
The type of permit, and whether you will need to supply insurance, will depend on the size of your party and the size of the street closure. PBOT waives some, but not all, fees for NNO parties. You will also 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.