Learn German: Kommt bitte her!

In German, hier refers to a fixed location (being in a place), while her implies movement towards the speaker. Because this command asks people to move from where they are to where you are, you must use the directional adverb her.

Why is the verb kommt and not komm or kommen?

This is a command (imperative) addressed to multiple people informally, matching the ihr (you all) form. If you were talking to just one person informally, it would be Komm bitte her!. If you were speaking formally, it would be Kommen Sie bitte her!.

Is her acting as a separable prefix in this sentence?

Yes! The full verb being used here is herkommen (to come here). In a command, the conjugated part of the verb goes at the beginning and the separable prefix (her) is pushed to the very end of the sentence.