Short answer
HubSpot's KB: users without permission can open Users & Teams and request access in HubSpot. Describe the task in What are you working on? and choose the Super Admin to notify.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Putting a user on more than one team is a Super Admin task, so start with the request.
3. Related
Inactive users and permissions history.