General Brazilian
encontro descontraído / confraternização descontraída
Most common usage in everyday Brazilian Portuguese
Rio de Janeiro
encontro relax / balada descontraída
Often uses English 'relax' in colloquial speech
São Paulo
reunião informal / encontro casual
More formal regions tend to use 'informal' or 'casual'
Portugal
encontro descontraído / convívio informal
Convívio is commonly used for social gatherings in Portugal