Trial accounts can't reply to tells, so yes, as obnoxious as it sounds the system was designed around mass invites, and the patch notes even recommend people to use player search for it.
Now this thread will further devolve into people attacking the network without being part of it.
OP's suggestion is good, mentor sends invite, novice gets a window explaining what is the novice network and if they want to join, they can y/n.
Also add a "leave novice network" button instead of having to type a command.
EDIT, READ BEFORE REPLYING. For any other person who wants to quote me: Trial accounts can't reply to tells, you have 50-60 new sprouts that are potentially trial accounts, how do you counter that?