C
You could, but you'll miss out from the things in the list I linked. 
It looks fairly straightforward to use:
A RESTful JSON-speaking API allowing you to write things to NodeBB - nodebb-plugin-write-api/routes/v2/readme.md at master · NodeBB/nodebb-plugin-write-api
GitHub (github.com)
(POST a username and (optionally) a password and email to the endpoint; job done).
@thomas0 It's worth digging into the code a little bit, more understanding will help when (or if.. 
) things go wrong. Just as an example, here's what a typical user object looks like in NodeBB:
{ "_id" : ObjectId("5aa26e9218d8783123dcb189"), "_key" : "user:1", "username" : "MyUsername", "userslug" : "myusername", "email" : "[email protected]", "joindate" : 1520594578043, "lastonline" : 1520602862371, "picture" : "", "fullname" : "", "location" : "", "birthday" : "", "website" : "", "signature" : "", "uploadedpicture" : "", "profileviews" : 0, "reputation" : 0, "postcount" : 1, "topiccount" : 1, "lastposttime" : 1520594579361, "banned" : 0, "status" : "online", "uid" : 1, "password" : "$2a$12$SnwLqMV2.g8ADjLEV.2tK.hGGD1o85rm4Tb/aj2twrTuU5BewqMCW", "passwordExpiry" : 0, "groupTitle" : "administrators", "rss_token" : "2e3df2a7-f579-431f-b5ad-fgh52b995b1c" }
(Just making the point you'd have to create all that extra data too, that the write-api plugin would do for you).
Actually, it's more than I thought. See this list from @PitaJ
This is a probably incomplete list of what needs to be done on user creation: get nextUid create userslug make sure userslug is unique increment uid save al...
NodeBB Community (community.nodebb.org)
I guess the problem for support (and anyone here) is that it requires knowledge of your Wordpress install and plugins to offer a solution.
I can't help you at the moment, but I was planning on experimenting with integrating NodeBB with a custom-built site of my own over the weekend, both of which I want to authenticate users with SSO. So I'll report back with how it went/if I find anything that might help you. 
@thomas0 Sounds like the way to do it.
From what people have been saying here, just make sure you use the plugin-write-api. Makes sure everything regarding user creation is taken care of.
It might be nice to also send a password reset email rather than plaintext password (makes sure they'll change it too, I guess!).
I think you're going to struggle migrating NodeBB -> BuddyPress. I don't think either platform has a large enough following to have an actively maintained migration path for either direction. 
I think your efforts/costs would be better focused on solving the shared-user-accounts problem between the two sites. It can be done.
@arkoma It looks like nodebb defaults to running as production, unless you pass some other value to NODE_ENV, so that line will have no effect.
Node.js based forum software built for the modern web - NodeBB/app.js at master · NodeBB/NodeBB
GitHub (github.com)
May be faster for you to figure out the correct credentials using the CLI:
mongo --username <yourusername> --password <yourpass>
Also worth attempting to connect with no credentials, on the off-chance you haven't correctly enabled authentication.
mongo
If you can't figure out the correct credentials, you can set up another set with this official doc.
@alexsir Yep, I guess that's the port NodeBB is configured to listen on my default. At the end of that guide, there's a link to another guide that explains how to configure nginx as a proxy - which would allow you to either configure a domain to point to your NodeBB process, and/or omit the port from the URL.
This guide takes you through the steps of deploying on a DigitalOcean Droplet (VM), after which the VM would be accessible via IP.
This time everything should be fine and we have successfully installed NodeBB on DigitalOcean. Your forum will be available on http://<droplet-ip=address>:4567
I haven't applied your CSS to my own install, so I haven't tested your specific code, but; if you want different behaviour for different medias (desktop/mobile), you can play with media queries.
e.g.
/* Smartphones */
@media (min-width:320px) {
body {
font-family: "Courier New", Courier, monospace;
color: #ffffff;
background-color: rgba(0,0,0,0);
}
html {
background: url(https://9854.moe/assets/uploads/system/background.png) no-repeat center center fixed;
background-size: cover;
background-color: #000;
}
}
/* Desktop */
@media (min-width:1281px) {
body {
font-family: "Courier New", Courier, monospace;
color: #ffffff;
background: url(https://9854.moe/assets/uploads/system/background.png) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
}Not to be rude/judgemental, but you will need some technical understanding to successfully deploy and maintain your own NodeBB installation.
The official docs are a great place to start, but they won't hand-hold you through registering a domain, configuring DNS, etc.
If you don't want to tackle that, NodeBB have paid plans that they will manage for you.
I don't have an install up and running at the moment, but this setting does appear to be configurable:
User.sendInvitationEmail = function (uid, email, callback) {
callback = callback || function () {};
var token = utils.generateUUID();
var registerLink = nconf.get('url') + '/register?token=' + token + '&email=' + encodeURIComponent(email);
var expireIn = (parseInt(meta.config.inviteExpiration, 10) || 1) * 86400000;
src/views/admin/settings/user.tpl: <input type="number" class="form-control" data-field="inviteExpiration" placeholder="1">
So I'd look for it somewhere in admin user settings.