You're clearly making a ton of money so I'm sure nothing I say here will have any bearing on your goals. I know, I'm just some dude who writes books and doesn't know anything about how real companies make money. You can totally take my feature request and laugh all day about how I don't know anything about startups, valuations, or websites. That's cool. Your loss. I'll just work around this one tiny glaringly obvious feature that'd make me some money, and you gobs of money.
All I want is a checkbox when I create my "team" that reads:
"Members pay $_____ to join."
What? Why would I want that? You see, I want to use Slack to teach people all over the world how to code. Right now I'm going to charge them myself and send them invites and deal with all the bullshit of membership. I'd rather just hang out in the Slack and answer questions and have them pay you so you pay me. I'm sure you have all the gear necessary to do this so I don't need to. Just need the ability to invert who pays and it's nearly perfect.
I promote it, I do the hard work of being in there, and you get to skim your $6.67 or however much the plan costs off the top of the signups. You give me a place to send them, and when my students join they punch in the required credit card and then they're done. You then give me the difference and we're good.
Easy right? You literally have every single feature I want to make this work. I just don't want to deal with the bullshit of payments and invites and booting people who don't pay and all that crap. You already do that. Me doing it is just stupid.
I have the clout and the skills. You have the platform. If anyone at Slack wants me to come down and talk to them about this I'd be glad to do it. There's a few other features that could help in this kind of use case, but really this is all I need to get started.