Create an easy new social event and invite parents - what about a start of term coffee or pub night? No organising, no fundraising; just a get-together to start the ball rolling.
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 you post regularly on the Parent Board, so that parents get used to checking the site. Whole school posts aren't emailed, but group (ie year, class or parent group) posts are.
Use the Classlist blog for inspiration and share links to stories with your fellow parents.
Set out your guidelines for how parents should use the site: here are Classlist's Community 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.
Have a clear-out and post some stuff you would otherwise be taking to the charity shop on the Marketplace - outgrown bikes, coats and roller blades, or those unused gadgets cluttering up the cupboard and books you will never re-read. You can ask for payment or give stuff away for free.
Send an Announcement, which allows longer messages to be emailed out and will go to all members and any invited parents you have invited by email in Admin Mode.
Create event pages 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 more parents have joined.