You can do that from the user's profile and click on the three dots under their name, and choose the Subscribe or Unsubscribe menu item.
You can also add / remove them from any lists you have from inside the Marketing tab by either clicking the green Add List button, or by hovering over a list and clicking on the X icon:
You can also subscribe users in bulk from your current Contacts tab by marking them and clicking on More. Then select Add Contacts To List and select to which list you want to add them. This will subscribe and add the contacts to the selected list.