systemd 232 introduced a really cool feature that allows running as a dynamically allocated user with DynamicUser=yes.
Both group and user are set to a free ID in the range 61184…65519. E.g.:
$ sudo systemd-run -p DynamicUser=yes -r -t id
Running as unit: run-re94d829d72f74046a1f402348bef2487.service
Press ^] three times within 1s to disconnect TTY.
uid=61791 gid=61791 groups=61791