Group progress for Elite Dungeons

A couple clanmates and I were doing the Temple of Aminishi elite dungeon and we all had to leave before we could kill Seiryu. If we rejoin in our group, will our progress in the dungeon be saved? Or will we have to start over.

How does group progress work in the Elite Dungeons?

According the Runescape Wiki, when you exit an Elite Dungeon or are killed, your progress is saved at certain checkpoints. Assuming all members of your clan had each reached the checkpoint during play, then you ought to be able to re-enter the dungeon and continue from the last checkpoint. From the wiki:

Your progress will be saved so you can head back in and continue where you left off, skipping to any unlocked checkpoints.

Just to add to the accepted answer, there is a bit more going under the hood when all members teleport out of the Elite Dungeon (this applies to all three Elite Dungeons).

If one member teleports out while another is still in the dungeon, the member is free to rejoin the dungeon at any time and use the chests to teleport to each reached checkpoint. If all members teleport out, then every monster that is currently aggressive towards any player despawns (*) from the dungeon (there are exceptions, such as only half of the Arhats in the room before the first ED1 boss). Yes, you can all rejoin the Elite Dungeon while saving your progress, as long as the leader when first entering does not reset your group progress.

* When they despawn, no experience is gained nor are any drops obtained. This is a useful tactic if you're being chased by many monsters and need them to disappear - but all team members need to teleport out.