If I attack with Anafenza, the Foremost and one other creature do I get to place a +1/+1 counter on the other creature or does it have to be tapped before combat?
Yes, you can put the +1/+1 counter on any creature that is tapped, including a creature that tapped to attack. The situation plays out like this:
- You declare which of your creatures are attacking, including Anafenza and another creatures. You tap those creatures.
- Anafenza's triggered ability goes on the stack, and you choose the target. At this point, you can choose any other tapped creature, including one that just tapped to attack.
- The triggered ability resolves, and the creature gets a +1/+1 counter.
During the declare attacker step you chose with creatures attack. Then after you chose all creatures who attack this turn, you put Anafenza's trigger on the stack.
While putting the ability on the stack, you have to chose a legal target, which is another tapped creature. This can be a creature tapped prior to attacking or a creature that you just declared as an attacker. If no target can be chosen, the ability cannot be put onto the stack.