Hi Ralph,
It's a bracket (some call it brace, but we usually reserve that name for the "{" shape - I say to-may-to, you say to-mah-to
)
If you look in your screen shot, the button on the far left is the braces/brackets button. To add one to your own score, just click that button, then choose which shape you want. Click in the score on the top most staff you want to include, then drag the bottom "handle" down to include the last staff you want to include.
Fwiw, you can click nearly any element in the score to change the tab and palettes to the element's associated palette and tools. Eg. if you click on the bracket in your score (actually click-dragging across it from left to right is the easiest) then the brackets tools will show on the palette.
You'll also notice that the "connecting" barlines are not present between bracketed staves, so it's also useful for choral work where you don't want barlines interfering with the lyrics.
Enjoy
Sherry