Learn Dutch: Zij zijn ook samen. - They are also together.

We can tell by looking at the verb! The verb is zijn (are), which is the plural form. If the sentence was about "she", it would use the singular verb form: Zij is (She is).

Could I say Ze zijn ook samen instead?

Yes! Zij and ze both mean "they". Zij is the stressed form, used when you want to emphasize who is together. Ze is the unstressed form that Dutch speakers use in everyday speech when no special emphasis is needed.

The verb here is zijn (are). Isn't zijn also the dictionary form of the verb "to be"?

Yes, it is! In Dutch, the present tense plural verb forms (for wij, jullie, and zij) are exactly the same as the infinitive (the dictionary form) of the verb. So zijn means both "to be" and "are".