doesn't that mean: they? not are YOU coming to dinner?
"Ustedes vienen" means "you [formal plural] come/are coming". The pronoun ustedes is used as a formal you plural (as opposed to the informal vosotros); but the conjugation of the verb is that of the "they form", so:
Vosotros venís a cenar.
Ustedes vienen a cenar.
Here is a article that will be useful to read talking about this. Have a look.
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