"Prendi l'ombrello con te!"
Translation:Take the umbrella with you!
It's imperative, the pronoun is not needed. Like "Do the dishes!" or "Stay away from fire!"
www.italian.about.com/library/weekly/aa011900a.htm - I struggled with this too. See the link for a good explanation of the commands for -are, -ere and -ire verbs. for singular and plural commands. I'm still struggling with it ... but this link should help.