One Player Barline?

How do I insert a barline for just one player, instead of the whole score?

Hi, if you want to insert a bar line for just one player, it means you want to give that player an individual meter.
Select the first note after your targeted bar line, click Shift-M (for meter input), input a new meter and confirm with Alt-Enter. The Alt key ensures the new meter is for that one player exclusively.

No, that’s not it. The meter is the same for everyone. But I want some specific players to have double barlines and not others.

In order to have different barlines, staves first need to have an independent time signature. This is because Dorico treats barlines as part of time signatures (because without time signatures, there is no logical reason for a barline in most cases).

So even if you don’t want these staves to have a different type or value of time signature, you still need to give them an independent time signature. Just re-input the prevailing time signature at the point where you want these individual double barlines. You can hide the independent time signatures so they don’t appear in the music.