In my experience, signup rate should not be important in building a successful community. You can have
either:
A) plenty of people signing up but not participating in discussions
or
B) few people signing up with strong intention to participate in discussions
E.g. you can have 1 active user per 100 sign ups per month (1%) or per 10 signups (10%) – the percentage is different, but you still have 1 active user.
In our community signups happen organically – partially thanks to the Discourse’s form that appears at the bottom for anonymous uses after they spend some time reading, partly because of the content that engages and invites people to say something.