"F" is the default assist key, and "U" is the default "interact" key. So you can have your slaves keep your main targeted and if your main targets the quest giver/object you can send "F" to the slaves to get them to assist your main, and then send "U" to get them to interact with the target. I believe there is also a way to bind a key to assist fellowship member 1, 2, 3, etc. if you don't want your slaves to always have your main targeted.

Personally, I like to have the slaves keep my main targeted all the time and use the skill forwarding option.