The verb “ser” is used to describe traits that are permanent, to describe an object, people, animals, to indicate someone’s occupation. Further, in Spanish “ser” is also used to talk about time, locations, etc.

The verb “estar” is mostly used to express temporary situations.
To express someone’s condition, mood, feelings in a specific moment. To indicate where someone is, how is the weather at a specific moment, and for ongoing actions.

The subjects are “estudiantes, profesores y otros” (ellos/they).
From the context of the text and the previous sentence, we can tell that they are describing “un club” (a club).
In the next sentence, they are telling us who the members are. We will use the verb “estar” because they are describing an ongoing situation, the members are “interesados en hacer teatro” (interested in doing theater). They are interested for a longer period of time.