Follow the upgrade instruction provided in the upgrade.
When you come to the merge instructions for /etc/groups, just hit [enter], like it says, and it will put you into an editor to merge those files. It will show what you currently have, then the new 13.1 file. The editor uses vim commands. I just had to delete one line in the new file, which was a duplicate of the old, but the old had my configuration in it (in my configuration, I had my username added to the video group and the upgrade did not). After you have made your edits, type
:wq
to exit the editor (vim) and it will save the merged file and continue with the upgrade.
At the end of the upgrade, there will be further instructions and you will need to reboot then continue the upgrade, as noted in the response by Maurizio.
This does not preclude you from going back and editing /etc/groups after the upgrade is finished, like you would normally do to edit your groups configuration in FreeBSD.
I highly recommend you backup your data before doing the upgrade. It worked flawlessly on my Nomad laptop, but killed my NAS, which uses older hardware. I’m still trying to rescue my NAS from a kernel panic page fault issue.