OneOfUs is a lightweight addon to help you relieve your fellow players of their un-guilded plight. This addon was inspired by the now-defunct SuperGuildInvite and its GuildRecruiter successor, but it is a complete rewrite focused on simplicity and efficiency.
This add-on works well in both Retail and Classic WoW.
NOTE: As of 8.2.5, all passive player scanning has been broken by Blizzard. It is no longer possible to scan for players in the background without some user interaction. This add-on has been adapted accordingly.
There is no longer a UI. Instead the add-on is entirely command-driven, and you can push the scan/invite process forward using a single command:
/oou step
This searches for the next small batch of unguilded players it can find and queues them up, and it also invites a single player from the front of the queue. You essentially want to spam this command as much as possible for maximum efficiency. There is no start/stop operation. Only step, step, step.
I recommend having a single macro dedicated just to this command which you can hit periodically, while also shoving the command into other commonly used (e.g. combat ability) macros. I tend to macro all of my class abilities, and so I also just shoved an /oou step line into all of the macros on all of my characters. Great for grinding in classic or doing dailies in retail.
Finally, If you want to see what the add-on is doing, you can use:
/oou debug
to toggle debugging mode on or off. This is very noisy for your chat frame when turned on, so you probably don't want to leave it on forever.