So I'm already pretty sure I know the answer to this, but I just wanted to be 100% certain.
My guild raids 2 days/week for 5 hours total, so we're probably going to extend this week (and maybe next week?) until we kill Lei Shen.
Last week we reached Dark Animus before we ran out of time, so we're going to extend and begin on him this week.
So my question is, before our raid starts on Tuesday, can we have people join early pugs/guild groups and clear the first few bosses, and then join our raid when it is our guild's raid time, as long as we still have someone from our guild's run that won't do an early pug and can still extend the lockout?
On normal mode, everyone will be moved to the furthest progressed lockout in the group, so yes, you can have people kill a few bosses as long as at least 1 person does not get saved to a new throne and can extend the lock from the past week. As soon as you are saved to a new week's lockout though, you cannot extend an old lockout anymore, so be absolutely sure you know who is responsible for saving their old lock.
Rules are more complicated for heroic locks. Basically heroic lockouts have unique ID's. Once you're saved to a heroic lock, you cannot join any other heroic raid ID for that week. Players with only normal mode locks can join heroic locks (assuming compatible progression), but will then be unable to leave and join a different heroic ID in that week.