This module is similar in purpouse to mod_groups, for when you want all users on the server to be in each others roster.
Upon login, this module will add all currently logged in users to the logging in users roster.
Just add it to the modules_enabled, after that there is no further configuration.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_roster_allinall
For earlier versions see the documentation for installing 3rd party modules