Right now on Ubuntu they use ConsoleKit for managing logged-in users, but ConsoleKit is no longer maintained. The functionality of FreeDesktop.org's ConsoleKit has since been integrated within systemd. With no longer having upstream maintenance provided to them, the plan is to switch to logind. However, the logind component can mostly work without a full systemd-based system, so that's the only part they're looking at using for the future of Ubuntu.
The systemd-logind service is responsible for tracking users/sessions, managing user log-ins with creating control groups for user processes, PolicyKit-based access for users, multi-seat management, session switch management, and other features.
Logind does carry a dependency on the systemd cgroups model and the PAM plug-in, but Ubuntu developers are looking to work that into Ubuntu. Moving to logind will also allow them to utilize a newer version of udev, which has also been since integrated with systemd.
More details on the Ubuntu migration plans to systemd are covered in the video below from the online Ubuntu Developer Summit that happened this week.