From the Manual:

on Use multirests.

Sibelius does this automatically for you in parts. There is also a special 
shortcut for switching Use
multirests on and off - Ctrl+Shift+M or xXM.

Sibelius automatically splits multirests at time signatures, rehearsal 
marks, key changes, clef
changes, tempo marks and so on. If you need to split a multirest manually, 
see Forcing a multirest
to split below.

Creating a multirest
Creating a multirest is the same as creating lots of single bar rests: 
choose Create > Bar > Other;
type in the Number of bars you want, click OK, then click where you want to 
put the multirest.

From Dan:

Here's some more info on multi-bar rests.
 
By default, multi-bar rests are turned on in the parts, so you don't need to
worry about them there. Full scores don't normally have multi-bar rests but
you can use them by checking the use multi-bar rests option in the layout >
auto layout dialog.
 
If you're reading a part or score with multi-bar rests turned on, you will
notice that navigating gets more interesting. To summarise, Sibelius turns
all of the resting bars into one bar that will contain the multi-bar rest.
Sibelius access cannot identify the multi-bar rest, however you will no it's
there because pressing RightArrow will take you straight to the next bar
after the multi-bar rest.
 
There's one interesting situation which arises when multi-bar rests is
turned on in a part. If the part has no music in it yet, it will appear to
only contain one bar. This is because Sibelius has written out the entire
part as a multi-bar rest.
 
If you need to add something to a bar which is inside a multi-bar rest, you
must first turn multi-bar rests off. Pressing Ctrl+Shift+M toggles this
option and will reveal the resting bars again. Once you've added what you
need, press Ctrl+Shift+M and the multi-bar rests will be put back.
 
Multi-bar rests are a good example of how dynamic parts work. The idea is
that you create the music in the full score, letting the parts format
themselves. You only go into the parts when the music is complete, and then
only to modify the layout.