When integrated with Salesforce, you'll need to import new users into HubSpot from Salesforce - rather than creating them via the regular email process.
Why?This connects the HubSpot user with the Salesforce user. If you do not do this, the Salesforce User ID will not connect to the user in HubSpot. To fix it after the fact, you'll need to submit a HubSpot help ticket with the User ID of the Salesforce user and the user's email address in HubSpot so they can manually map this connection.
How to create a Salesforce user in Hubspot
Go to "Create user" in HubSpot under Settings > Users & Teams
Select "Salesforce Records" in the bottom right under the Email field in the create screen