You’ll need to do things.
First, lock-down the site so that only logged in users can access it.
I just added a basic conditional to check if a user is logged in. If they are not logged in, they get redirected to my register page (slug is /join).
This is done through a bp-custom.php file that sits in my plugins directory.
Here it is: https://pastebin.com/rgJ6BQc8
Next you need to add a friendly message and login form to the initial registration page. This gives users a heads up they have to register if they haven’t already, and it provides an opportunity for returning users to log in.
I did this by creating a file called “functions-custom.php” in my Frisco theme directory. Then I went to Frisco theme options and enabled the custom functions file. Here’s the code from that file.
https://pastebin.com/wss8Fpdn
You’ll need to add some custom CSS to style the welcome message/login.