Groups in Microsoft Stream let you choose a set of people that you wish to collaborate with and set up a collection of resources for those people to share. Adding members to the group automatically gives them the access to the videos and channels within the group.
If you are an Office 365 customer, groups in Microsoft Stream are built using Office 365 Groups. When you make a group in Microsoft Stream it creates an Office 365 Group that can be used across Office 365, giving the group an email address, calendar, site, etc.
For more information on how groups work: Overview of groups & channels
Create a new group
Sign in to the Microsoft Stream portal.
Select Create > Create a group from the top navigation bar.
In the create group dialog, give a name and description for your group.
Check or uncheck Make this group copmanywide to determine who has access to the content inside your group. When this box is checked the group and videos you add to it to be viewable by everyone in your organization. When this box is not checked your group will be private and only the members in the group will be able to see the contents of the group.
Add group members to your group. You can only add individual users to groups. If you would like a member to be an owner of the group check the box in the Owner column to upgrade their permission.
Check or uncheck Allow members to contribute to determine if the members of the group should be able to contribute to the group by adding/modify videos and channels, or if they should just be viewers of the group only.
This setting only applies to Microsoft Stream, it does not apply to the Group when used in the rest of Office 365.