Microsoft has begun to roll out the ability to add anyone as a guest in Microsoft Teams. This means that anyone with a business or consumer email account, such as Outlook.com, Gmail.com or others, can participate as a guest in Teams with full access to team chats, meetings and files. All guests in Teams are covered by the same compliance and auditing protection as the rest of Office 365, and can be managed securely within Azure AD.
Microsoft Article - Add guests to a team
Video Guide - Adding Guests in Microsoft Teams