Hello @MelissaS246 ,
It's likely that they're running into some kind of network or device-specific connectivity error. I'd suggest cancelling those contacts' pending registrations, then asking them to re-register in either a different browser, in Incognito/Private browsing, or on a wholly different network than what they're currently using (e.g. using data instead of a work or home wifi). Other basic connectivity troubleshooting on their end may resolve the issue.
If you have a number of contacts still having issues, even after performing the workarounds, then please call our general support so they can provide more advanced troubleshooting live with you.