Toyhouse has been in invite-only alpha for half a decade now, and at this point, I've seriously given up hope it's ever going to develop beyond that state.
I think the site owner has achieved a balance of users + revenue + server capacity/expense which provides them with a comfortable living, and they're just not interested in potentially sabotaging that balance by opening the floodgates. I kinda respect that. Prioritising sustainability over constant growth and wealth is laudable.
To answer your question, you need to receive an invite code from somebody who has a premium account. They receive two per week. Toyhouse claims to generate codes for regular users semi-regularly, but I've been a member for a few years, and I've never received one. I got my invite from a friendly stranger on DeviantArt ages ago. You could try their forums, though be warned, DA is an absolute mess these days.
There are premium users out there who are generous with their codes. You just need to keep searching for them until you find one.