Hi there!

Thanks for your query.

In general, Joomla allows the creation of new users from the backend. So, it doesn't need any extra extension for that.

You can have a look at the following blog about access control list if it helps: https://www.joomshaper.com/blog/complete-guide-to-joomla-role-based-user-access

And if you want to make info available related their query, then you can use search function I think: https://www.joomshaper.com/blog/joomla-search-explained

But about the message sending capability, what do you mean actually? I think you can use a contact form or form builder addon of our SP Page Builder through which your clients can send emails to you!

Best Regards