Learn German: Was tust du für die Mannschaft? - What are you doing for the team?

Yes, für always takes the accusative case! The word Mannschaft (team) is feminine. In German, the feminine word for "the" is die in both the nominative (subject) and accusative (object) cases. So it is in the accusative case here, it just doesn't change its shape like masculine nouns do.

Why does this sentence use tust instead of machst?

tust comes from the verb tun (to do). In German, tun and machen (to make/do) can be very similar. tun is used more for general actions or behaviors, while machen is often used when creating or producing something. For a general question like "What are you doing?", both Was tust du? and Was machst du? are commonly used and mean the same thing.

How do you conjugate the verb tun?

The verb tun (to do) is slightly irregular because its stem is just tu-. To say "you do", you add the standard -st ending to get du tust. Other common forms are ich tue (I do) and er/sie/es tut (he/she/it does).

Is there an easy way to remember that Mannschaft is a feminine noun?