Yes, there is a command. It's something like /target_group_member #. Where # goes from 0 to 4, with 0 = F2, 1 = F3, and so on. I have a hotbar like that for quick curing (I find it's faster to hit alt+ a number to trigger them than it is to actually move up and click on a person).
The / command may not be quite right with the _'s, but if you go in game and type /target_group and hit tab it should complete it for you (if it doesn't, remoe the _).
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