"I present you to Yossi."
Translation:אני מציגה לכם את יוסי.
You use את to indicate a definite direct object. Usually these come with a ה, but names are automatically definite. In the sentence "I present you to Yossi", the direct object is "you", so you'd say אותך. In the sentence "I present Yossi to you", Yossi is the direct object so you'd say את יוסי.