Adding an instrument mid-score

Directly below Hide empty staves is an option that starts "Don't hide empty staves..."

This will not hide empty stave in the first system. It is checked by default. Uncheck it and all empty stave will be hidden.

But to be clear: unless you have some unusual special reason to disable that option, you should leave it alone. The standards for published music - and pretty much any conductor or music director wanting to use the arrangement - will want all instruments listed on the first page, You shouldn't deviate from this except in the special circumstances where standard music notation convention warrants it (eg, a soloist or divisi parts that only exists for a few measures).