A prompter is someone who, during a live performance, waits in the wings and 'loudly whispers' the lines out to the actors onstage, (when they need it.)

I bet you're thinking, 'Great!  That solves a lot of problems!  We can have a prompter!  If someone drops a line, the prompter saves the day!'

Sorry.  Don't do it.  In fact, I highly recommend against it!  No one that I know of uses them.  For, at least, a couple of reasons.  

First, what makes you think that the audience won't hear the prompter?

Second, your actors will plan on relying on the prompter.  Maybe not consciously, but, at the very least, subconsciously, they will.

Nope.  Nope.  Nope.  Don't use a prompter.  I don't even mention prompters now.  However, if your group is new, or used to prompters, I recommend that you walk into the first rehearsal and tell them, 'we will not be using a prompter'.  After the initial 'wailing and gnashing of teeth' is over, the actors will realize that if they drop a line, no one will be there to bail them out.  They will work harder on their lines.  I believe that a little 'well-placed' pressure can be a good thing.