Almost always, this is not your network. Members sign in with their own UIID, so the problem sits with their identity or their device, and you cannot reset it for them — there is no member password for you to change.
Send them through this, in order:
Try another sign-in route. Depending on what they set up: email, their UIID or .uiid file, or a login certificate.
Use backup codes. If they saved them at setup, these reset access.
Consider what changed on the device. Biometric sign-in is device-bound. A new phone, a device reset, re-enrolled fingerprint or face, or an unsupported browser will all break the credential on that hardware. They re-enrol on the new device after signing in by another route.
Check they have a UIID at all. New members sometimes try to sign in before creating one. It takes about thirty seconds at uiid.me, free, in a browser.
When it is your side: if nobody can sign in, or invitations never arrive, look at your SMTP configuration first.
Still stuck? Ask them to write to [email protected] with their device, operating system, browser, what they tried and what happened.