Post a new event - what about a start of term coffee or pub night?
Write a post or two on the Home page wall e.g. welcoming people to the site and inviting others to post too. Make sure to post regularly on the wall, so that parents get used to checking the site regularly. If you have been using Facebook to post to parents, you can post here instead. Whole school wall posts aren't emailed, but group posts are.
Set out your guidelines for usage (see Site Guidelines). You may decide you would like to point your new members towards those when they join, by putting a post on the Parent Board, to avoid any future misunderstanding. Stick to these guidelines by letting parents know if they have gone over the boundary. Classlist should not be used to discuss people connected to the school or for lobbying the school.
Post in the community groups too, e.g. the Ambassadors and Class Admins group. The post won’t be emailed to anyone if there aren’t any other group members!
Send an Announcement, which allows longer messages to be emailed out and will go to all members and non-members you have invited by email
Create events for any PTA events that you already know the date for. When you create the events they will appear in the Events list, but no-one will be invited until you click 'invite' and select which groups you want to ask. This means you can save the event and then ‘send’ invitations out later (triggering emails), by using event reminder, when the majority of parents have joined.