If two or more players group together and one of them has a shareable quest, that player can go to the quest in their quest journal and share it with the group. Then any player in the group who has not already done the quest can accept the share.

Repeatable daily quests can be completed once per day, so that means you can do the quest today even if you've already done it before, as long as you haven't already done it today. So for a zone that has 6 delve dailies, that neans you can do all 6 delve dailies each day, rather than just 1 per day as would normally be true.