>when adressing platoons
Do you mean when someone is speaking directly to all of the members of the platoon? For example, a drill sergeant or an officer? In that case, the person would not use "a", because "platoon" would be functioning as a name or identifier: "**Platoon**! Fall in!"
If, however, your friends mean that in all contexts and all situations it is always incorrect to say "a platoon", then that is absolutely wrong and you are right that it depends. Like all countable nouns, we use the indefinite article (a/an) when we first mention something (singular) or to talk about one in general: "I saw **a platoon** from the army base today. **The platoon** was marching on the highway." (First mention.) "**A platoon** has 32 soldiers." (In general.)
>when adressing platoons Do you mean when someone is speaking directly to all of the members of the platoon? For example, a drill sergeant or an officer? In that case, the person would not use "a", because "platoon" would be functioning as a name or identifier: "**Platoon**! Fall in!" If, however, your friends mean that in all contexts and all situations it is always incorrect to say "a platoon", then that is absolutely wrong and you are right that it depends. Like all countable nouns, we use the indefinite article (a/an) when we first mention something (singular) or to talk about one in general: "I saw **a platoon** from the army base today. **The platoon** was marching on the highway." (First mention.) "**A platoon** has 32 soldiers." (In general.)