The GROUP STEALTH rule states that when multiple characters are sneaking together, you roll the stealth skill of the character with the lowest stealth rating once for the entire group.
This rule makes sense for cases where a party is sneaking through the kipple waste or down the hallway of an abandoned building; but it doesn't take into account the advantages of having extra manpower in a crowded environment when conducting surveillance; especially when a subject starts to move.
When following a subject, one investigator will keep eyes on the subject and the other investigators will try to stay out of line of sight of the subject while following the first investigator. When the subject makes a turn, the first investigator can signal to one of the back-ups to catch up and take over the lead so he can keep going straight in case the subject has noticed them and is watching to see if they make the same turn. There is a lot more nuance to the split second decisions being made while shadowing, but I don't think diving deeper into it than that is going to provide context that will translate into useful gameplay advice. (The French Connection has a scene that illustrates the techniques in action, but unfortunately I couldn't find any online clips of that specific scene).
A similar principle applies to tailing someone by car, especially on the highway. (It's harder to catch up to take over the lead with a car in a dense urban environment, because you can't push past traffic the way you can push past pedestrians on foot).
The most basic version of a house rule for SHADOWING in the city that I would put forward would be use the rating of the character with the lowest stealth as written for GROUP STEALTH, but allow each additional character on the tail to assist, contributing their skill die, up three additional STEALTH dice.
If I wanted to make it a little harder, I'd make the shadowing character with the lowest OBSERVATION rating make a roll and say the number of successes was the maximum number of assists, and if there were no successes that the tail was lost without the subject necessarily even noticing that they were being followed. This is because when you are prioritizing not getting burned as much as not losing the subject, you are always riding the razor's edge between staying far enough away that you don't get seen and so far away that you lose sight of the target. No assist on said OBSERVATION roll, because the subject will probably be lost when the character with weak OBSERVATION is in the lead trying to keep eyes on the subject by themselves.
To add to that, if the subject notices that they are being shadowed with a successful OBSERVATION roll opposed by the assisted STEALTH roll of SHADOWING group, they can attempt to discreetly lose the tail without advertising that they burned their tail by making their own STEALTH roll opposing the OBVSERVATION roll from the previous paragraph. If they can't lose the tail this way, than can try to run, or go to somewhere other than they originally intended to throw the investigators off the trail.