Looking for a user registration for two areas of my wordpress site

On my wordpress site I have two places where it will become necessary for a user to register (i.e. like placing a classified ad or posting an event) before they can make their post. I don't want them to have to register to view the site, but before they can post these two items as mentioned above.

Can anyone make a recommendation to me for the best way to do this registration process? The above mentioned items they need to register for are free but require registration for security purposes.

I hope you are having a great day!

Thank you