Log in to your Mailinblack personal space and go to the Management area.

Go to the Users and Groups tab.

Click on "Add a group", enter a group name, then choose the selection type for the users to be added to the group.

Then save.

It is also possible to add users via multiple selection types. Once the first selection is made, click on "Add another user selection":

And choose the new selection:

Save to finalize the creation of your group.