Adding a user to the wiki seems like it may take a lot of personal information, but it just requires the fields to have something in them.

A dokuwiki "Add user" form

For Real Name, just copy the username over.

For Email, just use username@example.com.

For the password, use a secure password generator (like the one in your password manager). Or something trusted, like the pwgen command on *nix systems.

Here's the filled form:

A dokuwiki "Add user" form, but the data is filled in

Once you add the user, send them a DM on Signal in a disappearing message with their password. Note that they can change their password in the Update Profile section up at the top of the wiki.