Have you tried submitting an issue to the GitHub repo? Either way, you'll need to supply much more information than that. What version of PMMP are you running? What OS? Any errors/output? Does this occur without any plugins added? Have you tried reinstalling PMMP, etc.
Try reinstalling PMMP? Is there any correlation in the people that can't join? Where are they located compared to the server? What host are you using? I understand I'm bombarding you with questions, but it doesn't seem like a PMMP issue, so this thread may not be much help.