From SharePoint Online, click on the App Launcher Icon  from the navigation bar

In the app launcher  click on the Yammer tile

Either:
-  Click on the + sign next to the Group Section header; or
-  At the bottom left of the page, click on Create a group

In the Create Group dialogue box choose Internal Group or External Group

Under Group Name enter a name for the new group

Under Group Members type in the names of colleagues who are to be invited to the group

If you are creating an Internal Group, in the section Who can view conversations and post messages? choose whether the group is Public Access or Private Access

If you are creating an External Group, in the section Who can view conversations and post messages? choose either Approved Members or Only Group Members.

If you select Private Access or Only Group Members you have the option to hide the group from the directory and search results. To do this, uncheck the tickbox.

Click the Create Group button.