All code rules apply to meetings and gatherings (meetings of multiple guilds). See The Code
Meetings almost always take place at the Nightfall fortress (Rhea's lot). They can occasionally happen at Blackspore as well, just not as often. Gathering areas may vary.
Meetings happen whenever Rhea decides to have one (she will inform you at least 30 minutes before; on skype or kik), but Nightfall has meetings every Saturday at 5:00 Pacific Time. (6:00 Mountain, 7:00 Central, 8:00 Eastern.) Sometimes there can be surprise meetings that only apply to whoever is online at the time.