In Sibelius it’s technically impossible to center a group name between two staves and also individually name each staff. You can read all about it in the Reference, as well as see various workarounds there that partially address the problem.
But what I’ll show you today is one method not mentioned in the Reference. The idea came to me from composer Dimitris Marinakis. It also comes with its limitations, but depending on your score, you may find it useful.
We’ll assume we want to transform the Flute staves shown here into a group, with two flutes on the top staff and the third flute on the second staff.
First go to the Home > Instruments > Edit Instruments dialog. We’ll be creating a new instrument specifically for grouped instruments. Do this by selecting No instrument (barline shown). Rename it something logical, like No instrument (for instrument groups).
Then click Edit Staff Type… Switch on the Used for Ossias checkbox. This will prevent the time signature from displaying in our special staff. Switch off Initial clef to prevent the clef from showing.
Close out of the dialog boxes and return to your score. Add the new instrument to your score by typing I and then select the newly created instrument. Position it in between the two flutes, in this case.
Adjust the brackets, sub-brackets and barlines by dragging them. You may need to create a new sub-bracket by selecting the staves and choosing Notations > Bracket or Brace > Sub-bracket. Modify the new “instrument” name by double-clicking in the blank space, and then modify the single-staff names. I didn’t show this here, but you’ll want to do this for both the first system and the second system of your score, so that Full and Short names are properly changed.
Finally, drag your staves into place. The major limitations of this method are that you’ll potentially have lots of extra staves in your score, and you’ll have to do a fair amount of dragging. But if the distance between your staves varies a lot from system to system, and you want fine control over where the group name is positioned, this might be the solution for you.