Org-Wide Teams in Microsoft Teams

Org-Wide Teams in Microsoft Teams let you create a single Microsoft Team that includes all internal users in your organisation. Microsoft recommends that you make some changes to the team's settings to cut down on excess noise and notifications.

Currently the feature supports up to 1000 users, however there are plans to increase that limit.

From Microsoft's notes: Microsoft Teams will support an easy way for everyone in an organization with up to 1,000 active users to be a part of a single team for collaboration with the new org-wide team type. With this feature, all internal users (up to 1,000 active users) will be added as members automatically.