Just wait about a day!
When you sign up for cohost, there's a brief waiting period before you can post. We have this in place as a spam-prevention measure, but we're activating all new users once every week day! You'll get an email once you can post, so don't worry about checking the site over and over to see if it's changed.
If you want, you can check your place in line in your settings page. As long as this number is under 1,000, you'll be in the next batch!