SSH to lxr.room11.org. The server requires public key authentication, contact a room owner to have your key added.
Check out the sources from git into /srv/sources. The directory name will appear in the OpenGrok web interface, so choose a sensible directory name! Please use https:// URLs for the remote where possible.
If you are adding multiple branches of the same project, check the project out into two separate directories. By convention, the directories should be named project-name@branch-name, e.g. php-src@master, php-src@7.2 etc.
TL;DR be patient, it will happen by itself.
In general it's best to let indexing happen automatically - it happens once an hour and unless the project you added is huge, it won't take very long.
If you need to manually trigger a re-index, execute /opt/room11-opengrok/bin/cronjob. This script has an flock guard, so if it returns immediately it means there is already an index in progress.
IMPORTANT: If you kill the cronjob script it will not stop the indexing process, because the actual indexing process is owned by the docker daemon. It will, however, release the flock created by the cronjob, and allow a second indexing process to start, which at best will burn CPU unnecessarily and at worst will corrupt the index. In other words, if you start an index then let it finish!
Add newly created users to the wheel group to give the sudo permissions. Only do this for people you trust!