It is possible to give Haverford and Bryn Mawr College students access to your courses. If a TriCo student is officially enrolled in your course, they will automatically be added to your Moodle course roster. To access your course, the TriCo student should log into their home Moodle site and then click the "Swarthmore College Moodle" link under the "Network Servers" block to access their Moodle courses at Swarthmore.
To add a Haverford or Bryn Mawr student that is not officially enrolled in a Swarthmore class, have the student log into their their home Moodle site and then click the "Swarthmore College Moodle" link under the "Network Servers" block. This will automatically create a Swarthmore Moodle account and you can enroll the student per the instructions on this page: How do I add or remove enrollments?