If you have a keen eye for detail, or even just a knack for breaking things, you can accelerate the development of the next version of MuseScore by testing in-progress previews and filing reports of problems you encounter. Good testing directly contributes to the stability of the software, and it's an excellent way for people of all backgrounds to make an immensely valuable contribution to the project!
Design and usability
People deeply experienced with MuseScore may lack perspective on how things appear to those starting fresh, or transitioning from a competing commercial product. Are you helping someone who has never used MuseScore before? Or are you new to MuseScore yourself? Do you have experience with other notation software? Contribute feedback on MuseScore's usability to help shape the software's future design.
MuseScore thrives on developer contributions in the form of both contributed code and patches. Helping out in development helps the project move forward and stay competitive, and it is the best way to ensure that MuseScore can do what you need it to do on your next musical project!
Do you wish to extend the functionality of MuseScore without actually reworking the core code? Write a plugin!
Help get the word out about MuseScore! Put your graphic design skills or marketing savvy to work in promoting MuseScore and spreading the word.
Learn more in the MuseScore marketing group
If you want to contribute but don't have the time to help out in any of the specific ways detailed above... If you just want to say "thank you" to the folks who have put work into making MuseScore what it is... If you want to ensure that MuseScore's infrastructure stays healthy and strong... why not consider a monetary donation?