It's very simple to add students to a Sign in with Google Classroom if they've already been added to your Google Classroom. Just go to the Classes tab, click on the Google Classroom, and click on the "Add/Re-Import Students" button. Then select the email address and Google Classroom, and all new students who are in the Classroom should be imported
Because of how Google integration works, you cannot directly add students to a Classroom you've imported from a Google Classroom without adding them to the Google Classroom first. However, if you absolutely need to add non-Google Classroom students anyway, there is a workaround. Create a new classroom and student, or find them in an existing classroom, then click the dropdown arrow.
Select move, and select the Google Classroom to move the student into.