I get asked these question a lot.
My company has external users accessing SharePoint. Do I have to pay for this license?
What is an external user?
External users are not employees, contractors, or onsite agents for you or your affiliates.
External users inherit the use rights of the SharePoint Online customer who is inviting them to collaborate. That is, if an organization purchases an E3 Enterprise plan, and builds a site that uses enterprise features, the external user is granted rights to use and/or view the enterprise features within the site collection they are invited to. While external users can be invited as extended project members to perform a full range of actions on a site, they will not have the exact same capabilities as a full, paid, licensed member within your organization.
So in short, you don’t pay for this user
Use Office Online for viewing and editing documents. If your plan includes Office Pro Plus, they will not have the licenses to install the desktop version of Office on their own computers.
Perform tasks on a site consistent with the permission level that they are assigned. For example, if you add an external user to the Members group, they will have Edit permissions and they will be able to add, edit and delete lists; they will also be able to view, add, update and delete list items and documents.
Create their own personal sites (what used to be referred to as My Sites), edit their profile, change their photo, or see aggregated tasks. External users don’t get their own OneDrive for Business document library.