Basic Microsoft Graph API Integration added to WordPress Plugin

We are Property Hive, a plugin for WordPress that acts as an estate agency CRM allowing them to manage contacts and calendars. We're looking to build an add on that integrates with the Microsoft Graph API so clients using Outlook 365 can:

a) Sync emails so correspondence sent/received via Outlook mail shows on the client record in Property Hive (read only access required)
b) Sync events so events added in property Hive show in the clients calendar (1-way sync only at present)

I already have the structure of the add on/plugin in place, and once the authentication bit is in place I can probably handle the rest. The bit I'm struggling with is authentication, and understanding the relationship between WordPress users and Outlook users (i.e. will each individual user need to authenticate).

To summarise, looking for a) consultation from someone experiences with MS Graph API / Azure Directory and b) someone to ideally build the authentication step of the plugin.