to be in a play
'Estar de juerga' is a Spanish phrase, but its translation is not 'to be in a play'. Rather, it is commonly used in informal contexts and it stands for 'to be partying' or 'to be on a spree'. It is used when someone is out having a good time, usually involving music, dance, and drinks.
Last night we were partying at Juan's house.
This phrase indicates that the speaker and their friends were having a lively time at Juan's place the previous night.
My friends and I are partying this weekend.
In this example, the speaker informs that they and their friends have plans to enjoy a fun and festive time together during the weekend.
The group was partying all night.
This sentence describes a group of people who spent an entire night celebrating or enjoying themselves, suggesting a very active social event.