How do you add another instrument?

Press i
See https://musescore.org/en/handbook/create-new-score#change-Instrument-se…

When adding an instrument, I find it convenient to go to the edit menu (found on the top left hand side of the screen) and select "instruments" from the drop down menu. From there you will get a selection of instruments from which to choose.

If you wish to change an instrument, I have right clicked on a staff within the score, selected staff properties, and then clicked on the "change instrument" button.

There may be other or better ways but these work for me. I hope this helps!