2 ways to look at this I think, as I've got something similar in my current very crude setup, where everyone hits /follow party1 so the main shammy tries to follow the second one in line. Currently I just hit the macro and then go back to movement keys, breaking follow.

1- There's some sort of [nomod] stopmacro command you can put in at the beginning so that the active window stops the macro before assisting or following. The macro experts can probably clarify.

2- the standard UI does not consider the main active char (the large unit up in the corner) as party1. One of the standard unit frames addons does this, so rather than 1 large and 4 smaller units (party-party4) you have 5 equal sized unit frames and party1-party5 works properly.