There is no "default" way that group games are run - it really depends on the GM, the game, or the players in question.
As you said, solo threads are games with only one thread - technically, a roleplay with three or four people, that only takes place in one thread, would be considered a "solo" roleplay.
For group games, additional threads beyond the OOC thread might include OOC character sheets with character profiles, and IC threads with different locations. For example, a boarding school roleplay might have one thread for separate rooms - kitchen, parlor, dorms, gardens, etc. But a roleplay of a futuristic sci-fi voyaging-and-adventuring roleplay might have threads for each planet, in each spaceship, etc.
Posting order is decided by the group. Some groups establish a posting order, others don't. It really depends on the nature of the game.
The best way to learn is to participate in a group game, though. :) I've really enjoyed my experiences with group games.