According to the Moodle Docs resource, "A meta course is a course that is... linked to one or more courses for its student enrolment... Adding or subtracting participants in a metacourse can only be done from the courses it is linked to."
As soon as 'Course meta link' is selected from the enrolment method drop down menu, Moodle will display a list of courses that you can link to (again, in the form of a drop down menu).
Repeat the entire process as many times as necessary in order to link to multiple child courses.
It is not possible to add or remove users in a metacourse, as the enrolments are defined through the relationship with the link courses. If course membership is incorrect, it must be corrected in the relevant link course(s).