If you want to create a new group, either select the People tab from the top menu or from within your organization menu, select Manage People. Then choose the Groups tab and Add Group.
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Name your new group and select Create Group.

Now that you have created your new group, the next step is to add people to it by selecting the +Add Members button. 

Here you can select your people from your organization not currently in that group - to add them to the group. Choose the people you need to add and then select the green Add button.