So mentees I've worked with have usually come as part of a program, such as MLH[1] or Outreachy[2]. Other projects I've helped with mentoring have come from college programs or professors[3].
I have no idea how hard it is to get through the filters to get these opportunities; I know generally we ask them to only send us the best and that's worked out really well.
[1] https://www.youtube.com/watch?v=fbsI8l-MaFA is an interview done on my podcast (by the other host, Tabatha) with CID, who works with me on OpenStack Ironic. A little over 2 years ago, he was an MLH fellow (https://fellowship.mlh.com/) working with me -- now he's a maintainer on OpenStack and working on my team. This is an exceptional outcome, but he's not the only person who has found success via MLH that I know.
[2] e.g. https://lists.openstack.org/archives/list/openstack-discuss@...
[3] https://lists.openstack.org/archives/list/openstack-discuss@...
Always happy to train up new packagers and maintainers.
Are they using their Matrix server to filter out drive-by/low-effort contributions?
Ask any questions on matrix.