Short answer
HubSpot's KB: connect an individual work email and install the Office 365 add-in to insert documents, meeting links and snippets while composing. Enrolling in sequences needs Sales or Service Hub Professional or Enterprise.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Desktop add-in users follow a separate article.
3. Related
Customizing the add-in and add-in crashes.