Notes out-of-range

This picture shows all of the notes you can select for the range of your instrument. The notes are named so that C is the lowest note in an octave (as implied in the picture since it on the left of each row). C4 is the note called middle C. It's written on the first ledger line below the treble staff and the first ledger line above the bass staff. From your pictures it seems the notes are always written on the treble clef, which is not unusual for amateur brass bands.

Your score pictures show all of the instruments in treble clef. I think this is the first problem. You have the wrong instruments selected. When you create the score the first time, you need to do it with concert pitch turned off, which is the default state. When this is done you will see the instruments with their various key signatures and what is on the page is what the musicians will see. The score you are looking at should be written this way. If it's not, then you need to tell your friend that's what you need. What would be better would be if he exported the score to xml or musicxml and then you could open the file in MuseScore like it was a MuseScore song. You would then be prompted to save it as a true MuseScore file when you save or close it. You would then need to do very little work to make it correct.

To create my first score if I were you, I would use the Brass band template. It has many of the instruments already listed that you will need. From there you can add or remove instruments using the buttons in the middle of the instruments dialog you see when you press i.

When you add instruments to your score press i, there is a search box below the instruments on the left side of the dialog. Use this to find the proper instrument and the correct clef. For example type "eupho" and you will see there are three Euphoniums listed. The Euphonium, Euphonium Treble clef and Euphonium Bugle. Look at the instruments and you will see the Euphonium Treble clef and Euphonium Bugle are both C instruments written in the treble clef. MuseScore won't distinguish between the sounds, so you can pick either of these. When you add the contrabass bugle, you will see it is written in the key of C on the treble clef. If you play one that is in the key of G, then change the transposition. In the dropdown box that defaults to 0-Perfect unison, the number indicates how many 1/2 steps the transposition is. Since G is five 1/2 steps below C, you will pick 5-Perfect fourth as the tranposition. If you find that the notes are an octave high, you can change the number of octaves from 2 to 1 to lower everything an octave. Of course if everything is too low, you can change it from 2 to 3 to raise everything an octave. While you're here you can fix the names of the instruments if you want to. The long name is displayed on the first system and the short name is listed on the rest.

Once you have your instruments set up properly, you can adjust their ranges if needed. If you choose the correct instruments, you should not have to do this much. The ranges are all written pitches, so it's less confusing if you make sure the concert pitch button is not pressed. If it's not pressed you will see different key signatures on the various instruments. Click the icons to the right of the amateur range and adjust the top and bottom notes as needed.

As I stated previously, the picture shows all of the notes you can pick for the various ranges of the instrument. Click the written pitch to select it and click OK to assign it. I would only worry about the amateur ranges, unless you make the ranges greater than the professional range, then you need to adjust the professional range to at least be equal with the amateur range. You will then have all playable notes in black, difficult to play notes in olive green and unplayable notes in red. Be aware that all notes will print in black, the colors are for information on screen only.

Once you have your band set up, save the score in the custom templates folder described at https://musescore.org/en/handbook/3/create-new-score#user-templates-fol…. On windows you can copy %HOMEPATH%\Documents\MuseScore3\Templates to the save score dialog and this folder will open. I'm not sure about other systems. You can then use the score from the new score wizard. It will be listed under custom templates, which is created only when you save a score there. The score will act like any other template and can be edited at any time if you find a mistake by opening it like any other score. This way, you only have to do all of this once if you are forced to manually enter the scores.

The Wikipedia article on transposing instruments is pretty good for getting an understanding of what you are dealing with here: https://en.wikipedia.org/wiki/Transposing_instrument