I run a small, and not very active Discourse site. I do both parts of administering the Discourse discussions, and running the back end computers that host the Discourse software. It sounds like you’re planning to pay for a Discourse site, which takes away the back-end parts. Those are actually pretty easy for anyone who is familiar with managing Linux systems.
Initial configuration of the message board itself will take time. That is making all the settings of titles, names, logos, categories, tags, permissions, and those types of things. If you know what you want, that will help a great deal. There is still going to be the time of playing with the software to make what you want into what exists.
Expect that step will take a minimum of several hours, but when you add in the time to familiarize yourself with how exactly you do things plan to spend 10+ hours. Lots of that may be on things like creating multiple versions of your logo in different aspect ratios. If you don’t care about that kind of detail, it might go much faster.
If you want different permissions for different groups of users you may have to create that. This is different than mods and stuff. It would be if you have people in the (riffing of your username) people in the “semi-colon” group can see the mid-sentence category, and those in the “question mark” group can see the end-sentence category. It’s best to plan this out ahead of time because…
Inviting all of the users is mostly easy. You can upload a csv file with a list of email addresses, and what each user’s primary group should be. Discourse will send out invitations. This step is extremely unforgiving. It’s just going to do it. If you have mistakes, like you created the group “semi-colon”, but have in the csv file “semicolon”, it won’t halt, it will send the invitation, but not put people in the right groups. Then you get to spend time manually adjusting 700 users.
Or maybe you just don’t do groups at all. Other than mods, I don’t think SDMB does groups.
Yeah, also the whole trust level stuff. On my end I adjusted it so new users can post, but as they rise through the trust levels they don’t get any new privileges. So like SDMB, I mostly subverted the trust system. I do use groups to give certain users junior-mod type abilities, like moving posts from one category to another.
To keep your content off search engines, you can set categories to only be readable by logged in users, and you can block self-creation of accounts. New users are invitation only, or need to be manually approved. Search engines will be able to find your site, but shouldn’t be able to index any of the actual content.
To me, the biggest thing is switch in feel between a message board, like Discourse or vBulletin, and a chat system like Signal. It’s really going to depend what type of interaction your group is going to prefer. Other options may be things like Discord or Slack.