Who dismisses the guest in Macbeth?

That depends on who you mean by the guest, and when.

In Act 3, Scene 1, Macbeth calls Banquo "our chief guest." Macbeth is the one who dismisses Banquo after that conversation, saying "Hie you to horse. Adieu, / Till your return at night."

If you mean the guests at the dinner party in Act 3, Scene 4 (after Banquo has been murdered and is appearing at the dinner as a ghost), Lady Macbeth dismisses them by saying "At once, good night. / Stand not upon the order of your going, / But go at once."