Answer:
se pusieron
The preterite tense is used to talk about actions that were completed at a certain moment in the past, that is, it talks about a specific time. The word ponerse can be separated into two words poner and se. So poner is an infinitive verb that means to put and se is the reflexive pronoun that matches the third person plural. To write the preterite form of the verb in parentheses we must conjugate the verb poner as pusieron for this matches the third person plural too. Also, the reflexive pronoun must come before this conjugation, therefore:
Cuando los animadores supieron que nuestro equipo ganó, ellos se pusieron a gritar.
