Sitelok is an add-on piece of software that will do exactly what you need. The owner is very familiar with RapidWeaver - he even built his site using it. On top of that, @vibralogix (Adrian) is very, very helpful. Read through this post for more information: Shout out for Sitelok - password protection & membership software
I laso have heard that a pre-eminent Stacks developer will soon be releasing a new product that makes integrating Sitelok even easier into Rw sites. Be on the lookout for an announcement soon.
Cheers,
Dave