Learn Spanish (Latin America): Mis amigos aún no han llegado.

Yes! Aún no and todavía no both mean 'not yet' or 'still not'. You can use them interchangeably. Saying Mis amigos todavía no han llegado would mean the exact same thing as this sentence.

In English, 'yet' goes at the end of the sentence. Can I move aún to the end here?

Yes, you can. It is very common to place aún no before the verb, as seen here. However, you can also say Mis amigos no han llegado aún, which perfectly matches the English word order and is completely natural in Spanish.

Could I put the negative word between the verbs and say Mis amigos han no llegado?

No, in Spanish you can never separate the two parts of the present perfect tense. The helping verb (han) and the past participle (llegado) must stay glued together. Words like no, aún, or ya always go before the entire verb phrase.

Why does aún have an accent mark?

The accent mark distinguishes it from another word. Written as aún, it means 'still' or 'yet'. If you write it without the accent (aun), it means 'even' (as in 'even my friends').