Just in case this helps someone else out there, I came up with two solutions: Macro 1: /macro TgtEny "target_name <playername>$$assist"
This macro directly targets a specific player by their name (replace <playername> with the actual name) and then assists their target. It's a more precise way to select a particular teammate, especially in a situation where there may be many players around. Works well if you typically play with a set group of team mates.
Macro 2: /macro Teammate "target_custom_near friend alive teammate$$assist"
This macro targets a nearby, alive teammate without specifying a name, then assists their target. It's more generalized and doesn't require you to know or input the player's name.
Cheers!