Welcome to the Community and thank you for checking my colleague's post. Just to confirm, has the user taken the steps outlined in this post? (check spam, verify spelling, allowlist noreply@hubspot.com)
If yes, could you please share or DM me the following information, so I can have a look on our end:
1. Email address of the user
2. Portal you are trying to add the user to
3. Confirm where in the process the user is getting stuck