Before you invite all the other parents - get your next event(s) set up
When you invite parents to PlanSocial, there ought to be something there for them to look at, or they might not bother coming back!
Add your upcoming events to the list, with some basic details and even some volunteering tasks if you already know what you need. You could even set up a task called "Offer to organise this event"! You never know who might fancy having a go...
Also a welcoming message on the News Message Wall is a good idea. Remember, don't say "We're trialing this..." Just use it! You'll only sound wishy-washy and put people off! :S