Hosting a Beatsaber lobby again tomorrow for anyone who's interested in that sort of thing.
It'll be 8pm CDT or 21 & 1/2 hours from this post.
To join the lobby use mod assistant to install everything under core as well as custom avatars, and mapping extensions. Then find the mp mod below. Merge to your folder then run ipa.exe
https://github.com/superrob/BeatSaberMultiplayer/releases

I know there was an anon who had trouble with the multiplayer plugin last time. Have you got it working since then?