Control access for new users and Captcha

There will be lots of spam users to the sites once it's online, so we have to a) install a captcha (modules captcha and re-captcha, see e.g. on the marbigen site) and b) restrict access (applications for new accounts have to be approved by an admin) . However, the latter might be time-consuming or impossible when we have the actual divers all at once creating accounts... Have to find a solution for that...