Verbs of perception (see, watch, hear, feel, notice) are followed by the infinitive without 'to' when the action being perceived is finished.
In your examples you used proper nouns....Sarah & Melissa.
Does it make more sense to you if you replace them with pronouns?
John sees her enter the house.
John sees her shake hands with Lloyd.
If the action is not finished, or the whole of the action is not perceived, verbs of perception are followed by the -ing form.
I heard Sarah singing in the bath (but I didn't stop to listen long!)
Hope this helps.