User Groups with NixOS

User groups are not created automatically with:

users.users."dendrite" = {
  group = "dendrite";
  isSystemUser = true;
};

# id dendrite
uid=996(dendrite) gid=65534(nogroup) groups=65534(nogroup)

You need:

users.groups.dendrite = {};
users.users."dendrite" = {
  group = "dendrite";
  isSystemUser = true;
};

which gives you:

# id dendrite
uid=996(dendrite) gid=994(dendrite) groups=994(dendrite)