Assuming you have their ownership key, you'll need to perform a breach. This will let you create a new pier from scratch.
What I'm not totally sure about is whether they'll automatically receive invites to groups they were in or something. Someone from the Landscape team can answer that.
You can test this flow without spending ETH by doing it with moons. Boot a moon, join some groups, delete the moon, on the planet run |moon-breach ~moon-name, boot the moon again (with the same keys) and see if it all works fine.