To claim a site using Friends Security, click the closed padlock icon that appears at the left end of the bottom bar. It will generate a synthetic name like Wecago and the words "Site Owned by: Wecago" will appear.
That will write a file into the site's status/owner.json (for a wiki at foo.example.wiki this will be ~/.wiki/foo.example.wiki/status/owner.json) like this:
{ "name": "wecago", "friend": { "secret": "long-hexadecimal-gobbledygook-here" } }
The name value can be edited to a name like "John Doe" and that will become the site owner name the next time the server process is started/restarted.
The secret value is what someone needs to paste in when they visit the site from a browser not logged in to it and wish to claim ownership. They click the closed padlock and get an alert prompting for this value. If they enter it correctly, they're in.