Learn Russian: Они тоже не здесь.

No, они (they) is used for any plural subject. Unlike он (he) and она (she), они does not change for gender. You use it for groups of men, women, mixed groups, or even inanimate objects.

Where is the verb 'are' in this sentence?

Russian usually leaves out the verb 'to be' in the present tense. You do not need a word for 'are' here. The sentence literally translates to 'They also not here.'

Why is не placed right next to здесь?

The negative word не (not) always goes exactly before the word it cancels out. Because we are saying they are 'not here', не must go directly before здесь.

Could I put тоже at the very end of the sentence, like we do with 'too' in English?

While Russian word order is flexible, тоже (also or too) sounds most natural right after the subject it applies to. Putting it at the end is understood but sounds a bit unnatural to a native speaker.