On some forums, non-expert forum posting might do the job for a bulk of the posts, but to attract real visitors there should be some experts posting. In other words, some people that know the niche beyond an amateur level. You might have to pay for the non-expert posts and pay even more for the expert ones, but it would be worth the cost. Note, though, having a bulk of non-expert posts might give it a feel that relates more to ordinary people.
As others have mentioned, give your guest a reason to sign up. Share quality content, offer incentives for new members, ask your friends to help post as your starting up.
When Reddit first started, they had their employees make a whole bunch of fake accounts and faked it till they made it with their fake accounts having made up conversations with each other. Might be something to look at for your forum.