After much work I managed to get my plugin working. On the plugin config page I called the database for a list of user emails, then presented three text boxes for users to filter this list down to create their monitor groups. I then saved the filter text boxes as strings rather than the list of users, and when a new bug is submitted used those filter strings to recreate the monitor group shortlist, checked reporter ID against that and if positive, assigned everyone in the shortlist to monitor.
If anyone ever has a question about this admittedly pretty simple process, don't hesitate to PM and ask me, however far in the future this is.
So now the problem I am going to tackle is that Mantis sends an email notification when someone monitors an issue that you are monitoring. If I have a "monitor group" of 18 people, the first person will get 17 notifications, the second will get 16 etc. That is a lot of email to receive all at once. Now I am going to poke around and find a way through the configuration options or the API to turn off these monitor notifications, either temporarily or permanently.
As always, if anyone can point me in the right direction it would be greatly appreciated!