Users will be able to message selected admin(s) only.
How to Configure:
Go to dashboard > Front End PM > Settings > Recipients. under Only Admin section you can set unlimited number of admins. Write Display as how you want to see in front end and that admin username. When some user will send PM to that admin, that PM will be sent to user of that username.
If only one admin is added and users can not send message to other user or group then “To” field will be hidden from front end. So that users have to fill only subject and message. That message will be sent automatically to admin.