Learn Portuguese (Portugal): É essencial vocês confirmarem a data. - It is essential for you to confirm the date.

Because there is no que in this sentence. If you use que, you need the subjunctive: É essencial que vocês confirmem a data. But as an alternative, you can drop the que and use the Personal Infinitive, which is what confirmarem is here. Both versions mean exactly the same thing.

How is the word confirmarem formed?

It is the Personal Infinitive form of confirmar. For regular verbs, you take the full dictionary form (the infinitive) and add personal endings. Since the subject is vocês, you add the plural ending -em to confirmar, giving you confirmarem.

Is it mandatory to include vocês in this sentence?

It is not mandatory, but it is very helpful. The ending of confirmarem tells us the subject is plural (either vocês, eles, or elas). If it is already obvious from the conversation that you are talking to 'you all', you can just say É essencial confirmarem a data. Adding vocês avoids any confusion with 'them'.

Could we put vocês at the very beginning of the sentence?