My understanding is that it is totally dependent on the invite order.
But that a given toon sees their own Mercenary in the F2 slot (with their own toon being their F1 slot).



Another option is to have two configurations.

I have either six toons in a group.
Or four toons in one group and two toons in another (purely due to level difference; this will eventually be three + three, each with 3x Mercs... as the alternate group).

My invite toons mapped key is on hot key: Control + I.
I've made two of these, which both use that same hot key.
One is set to not load on default.

I have the default Assist/Follow, based on Alge's Guide (and MiRai's Video) for Assist/Follow from any window for any game.
I have an alternate set of Variable Keystrokes, for when I'm in two group mode.

The hot key that switches from Mode A to Mode B, enables my alternate mapped keys (which are disabled by default) and disable the initial/default keys.
The hot key that switches back to Mode A from Mode B, does the opposite.




You could have two mapped keys, both with the same hot key to trigger them.
The default is your current version.
The alternate is set to not load at start up.

Then make a toggle key (or two keys, one for each mode).
Give it a PopUpTextAction, to let you know what mode you're in.

When you go into Mercenary Mode, the second (initially not loaded) mapped key is enabled for all characters.
And the initial (loaded, when the character set is loaded) mapped key is disabled for all characters.

The first one will target Slot 2, as your SK is there usually.
The second one will target whatever slot your SK is in, with Mercenaries in the group.
As long as you hit the correct toggle, for the mode that you want to be playing in...
The same hot key, will trigger the same action... and in both cases target the slot that your SK is actually in.