"hiermit" is usually something close to you. ie. you are lifting a rock and ask other people to help you.
"Damit" is usually used with something not close to you, or something you are only talking about at the moment. i.e. "I want to lift that rock over there, can you help me with that?" or "I want to lift rocks later, could you help me with that?"^^
Clicked on "helft" for the conjugation, but unfortunately it didn't include the imperative, so I just looked it up in my 501 German Verbs:
Imperative forms for helfen: hilf! helft! helfen Sie!
hilf! (2nd person sg. familiar) (Dude! Help me!)
helft! (2nd person pl. familiar) (Dudes! Help me!)
helfen Sie (2nd person singular or plural, formal) (Sir(s), please help me!)
I guess it must be informal second person plural - asking help from multiple people. (Read up on German Imperatives - I'm not sure what the best source is, Google's first hit, for me, was http://www.dartmouth.edu/~german/Grammatik/Imperative/Imperativ.html)