I use Ballstreams. I pay for it, but it's been worth it. Ballstreams has a Plex app and Roku app. I've had little to no issues with Ballstreams on Roku. Can watch any NBA game, can select which commentators I want to listen to 90% of the time, can start the game late if I want to, etc. Only got it because Dish dropped CSN, and I had no other option to watch the Celtics because of blackouts on League Pass. Wouldn't usually support something like Ballstreams, but professional sports leagues are dumb with their blackouts so meh. I would have paid for League Pass if I could actually use it for the C's without a proxy or some dumb workaround.
Ballstreams has an "invite" system, but it's not from the users like me. You just sign up and the website owners usually send out an email with an invite within a week...dunno if they still send invites out mid-season, but most of the people I know that use Ballstreams haven't had an issue getting an invite.