Short answer
HubSpot's KB: to resend, click Actions > Resend invite. If the invite was delivered but can't be found, send a reset password email. If the first delivery attempt fails, HubSpot blocks that address and you need HubSpot Support to fix it. Users who already have a HubSpot password can be added without an invite email.
1. Users
Active users in my demo portal, blurred.

2. IT filters
Ask IT to allow HubSpot's emails if invites keep disappearing.