If you need a group for a use case, create one in Yammer.
Create a group
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Select Create a group at the bottom of the Groups pane.
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Select Internal Group or External Group.
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Type in a Group Name.
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Type in the names or emails of Group Members.
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Select Public Access or Private Access.
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Select Create Group.
Add information and customize the group's look
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In your group, select Settings.
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Type a brief statement in the Group Description about the purpose of your group.
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To add a photo:
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Select Add.
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Go to the picture you want to use.
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Select Open.
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Select a color for the header.
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Select a Group Pattern to give your group some design and personality.
Source: Microsoft