We usually say travailler un instrument. Travailler le piano, or more idiomatic, travailler son piano. It can be a good translation when it's about practicing on your own, practicing scales for example. Musicians may also travailler together. Example: Ils travaillent ensemble une sonate pour le piano et le violon.
But when it's about playing together before a concert, we rather use the verb répéter. It tends to be an intransitive verb: Nous avons répété avant le concert. I don't think we would say Nous avons répété au piano et à la guitare.
Répéter conveys the idea that as a whole you're already prepared, you just have to refresh it so to speak.
In the examples above I'm not sure if any of those two verbs is suitable, though. I'd tend to say: J'ai fait du piano et de la guitare avec mes amis.
Faire d'un instrument is jouer d'un instrument, yes, but rather in the meaning of being able to play, because you usually practice the instrument, and by extension, I think we would understand it as "to practice", though it doesn't exclude playing together for the mere fun of it, it can be both I think, depending on the context. Faire is a cover-all!