How does invisibility work in combat?

In one fight I want 2 duergar and one kobold to fight my party. I want the party to see them all and then let the duergar become invisible. Once they are, do they need to roll stealth to sneak up on the party and does the party than roll perception in order to search for them? Or can I just go with the duergar up to one ranger and attack him (with surprise??)