Tip to grow membership: "register" link in welcome post.
My first CP site went live a couple days ago.
Most if not all registrations have come from a "register" link in a welcome post, rather than the register link in the default CP header.
I assigned every category (and pre-defined location tag) to this post so people can find it easily.
The post is just a couple sentences like "Welcome to sitename. Please register to begin listing your goods and services"... with the word "register" linking to the registration page.