You can add sub-containers in containers. The linked containers will be rendered as cards showing their metadata, so you can easily preview important things such as the number of over due and pending tasks, or phone numbers for contacts.
Creating sub-projects and sub-contacts
Simply go to any project or contact first. If it's being sorted "By Type", then click on the "+" button next to the Linked Containers heading to get started. If it's being sorted by any other mode, then click on the big "+" button on top and choose "Project or Contact" from the popup menu.
Timelines are linked, too
When sub-containers are linked, their time-sensitive items will also show up in the Timeline view of the parent container. So if you create a master project and link to multiple sub-projects, you will be able to see all the time-sensitive data, including the linked sub-projects, in the master project's Timeline view.