ISBoxer got you covered regarding follow (alt+F) and assist (alt+A) by default; those are dynamic and will follow/assist whichever character you are currently controlling.

However, currently there's a known issue for WoW retail after the recent patch, that partially breaks those functions.
Jabberie posted a workaround for it on the ISBoxer discord:

Quote Originally Posted by Jabberie

There is an issue with modifiers in World of Warcraft at the moment. Left and Right Modifiers are not being differentiated so FTL used with Follow and Assist do not work. In a team of 10, slots 2, 7 and 8 are affected.


If you are 5 or less, you can update slot 2 FTL modifier like this. You have to /reload ingame after export.
https://gfycat.com/hastyexhaustedbordercollie (change to HD in the lower right)


You can only use FTL up to 7 slots now. This is the 7 possible combinations you can use.
https://i.imgur.com/UjsLFPO.png


This is a WoW Client change and not something that is directly related to isboxer so we'll have to wait and see if it gets changed back.
https://us.forums.blizzard.com/en/wo...list/618955/24
That should cover everything in one post to link to.