An invite link opens /join, and that page says exactly why it did not work. Ask your teammate which line they see.
"This invite has expired"
Link invites last 14 days. Open Settings, then Team members, revoke the old one under Pending invites, and create a new invite.
"This invite is only for [an address]"
You filled in Lock to their email, and they are signed in with a different address. They can select Sign out and switch account, sign in with the address you locked it to, then open the link again. If they will always use a different address, revoke the invite and send a new one with that address, or with the field left blank.
"This invite link isn't valid"
The link is incomplete, or it has been revoked. Copy it again from Pending invites using Copy link — a link broken across two lines in a chat message is the usual cause.
"Sign in or create your account to accept this invite"
Normal, and not an error. They need a QuietStay login before they can join. After signing in or creating an account they come straight back to the invite.
If they had already started a guidebook by pasting a listing link, that half-made session cannot accept an invite. The page signs it out of that browser and asks them to sign in properly. Their draft is not lost.
"Something went wrong accepting the invite"
Have them open the link once more. If it repeats, send an emailed invite instead — that adds them to the team without a link to accept.
If it still will not let them in
Reply to any email from QuietStay with the address they are using, and it reaches a person.