Learn Polish: Czy często tu jesteście? - Are you plural often here?

In Polish, you usually drop the subject pronoun (like wy) because the verb itself already tells us who is doing the action. The -ście ending in jesteście clearly means "you (plural) are". You would only add the word wy (making it Czy wy często tu jesteście?) if you wanted to add emphasis, as in "Are you often here?"

How does jesteście fit into the plural forms of być (to be)?

Jesteście is the "you plural" (you all) form. The present tense plural forms for być are jesteśmy (we are), jesteście (you plural are), and są (they are). Notice how the "we" and "you plural" forms just take the singular word jest (he/she/it is) and add the plural endings -śmy and -ście.

Why is the verb jesteście at the end of the sentence? Could I say "Czy jesteście tu często?"

Polish word order is very flexible. Saying Czy jesteście tu często? is completely correct and natural. Putting the verb at the very end, as it is in this example, just shifts a tiny bit of focus onto the action of being there, but the core meaning doesn't change at all.

I've learned tutaj for "here". Is tu the same thing?