Whoa. Installed on my existing, populated Localhost dev-site.

Disregarding that the plugin installs a theme, which is also packing another plugin, it's interesting.

On an existing site, and without Buddypress or bbPress, it's quite rough-hewn, but is clearly striking out in new & intriguing directions.

It seems almost like an 'inside job' WordPress fork.
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In Admin, there is a new Menu-entry: Wikis. Make new Wiki; wiki Categories; wiki Tags, Flags & Settings.

I then selected my normal theme, with the UserPress plugin still activated. At first, it appeared to fail, but when I changed pages in Admin, it had switched.

The beloved/bemoaned Admin Bar vanished under UserPress, and returned with my theme.

The whole 'project' (it hardly seems right to call it a plugin) is too much of a hairball for my tastes, but it looks it will come apart fairly readily ... and in fact works, when only partly intact.