Mientras

Mientras
 

Should we use the subjunctive below after mientras, or does it need to be mientras que before the subjunctive is used?
¿Qué van a hacer tus amigos mientras están aquí en Zaragoza?

(My friends have not arrived yet; they're arriving next week.)

Many thanks in advance.

"mientras están aquí en Zaragoza" is correct -and the only option- in that example.

Though "mientras estén" and "mientras que estén" can be used in similar phrases, they have different meaning and in fact it's an advanced feature of the language.