The Office add-in can be distributed to users fairly easily, since no installation on the users device is required. In theory (if enabled) each user can just click on “Get Add-ins” in Outlook and install the add-in from there.
You can learn more about the detailed steps here: Installation Office add-in.
It’s also possible to deploy the Office add-in centrally via Microsoft 365 or Microsoft Exchange, which is explained in detail in the following:
If you want to deploy the Office add-in centrally to all or a certain subset for your users, there are two options. If required by the guide, please use this AppSource listing: Office Add-in in AppSource
Via Office 365 admin center
Please check this guide in the official Microsoft documentation on how to do this.
Via centralized deployment Powershell commands
If you prefer to use powershell commands, you can use this guide.
Exchange on Premise
You can deploy the Office add-in on an Exchange server centrally, by using EAC or Powershell. Please read the guide in the official documentation: Install or remove Office add-ins.