Should I use CentralAuth or shared tables?
Thanks. Do I just setup the Global User Groups on the main wiki, or do I add it to all of the wikis?
I would suggest to use a global file for easily updating if you are not using one already. You just need to add or remove the global group(s) in one wiki and it / they will be added or removed on every wiki. If you decide to setup GlobalUserGroups extension on the main wiki only, global groups must be added and removed there only. Or you set it up for every wiki but you just allow adding and removing on the main wiki by default so you can add and remove global user groups on other wikis too if you grant the rights to system administrators to do so. You should set it up for every wiki and add messages for globallyblocked group in GlobalUserGroups.i18n.groups.php. But in any case, you need to set the $wgRevokePermissions part for every wiki.
Do I need to add
to the php.ini file?
I saw it when looking at Extension:MaintenanceShell. I noticed that using cron jobs to run the update.php were unsuccessful in previous projects, and I wonder if it was because of that, or if that setting only affects Maintenance Shell.
The problem with this is that I haven't been able to run the update script.
I don't think so. Where should I look for it?