This presumes you have one volume, which is the default. "vol status" will give you a list. This will add any spare disk to the volume - you can also specify which one to add if you'd like.
Note that you may have to add more than one disk to a volume, if all the RAID groups in the volume are full. "vol add" lets you add more than one disk to a volume.
A handy trick if you run out of space is to reduce your snap reserve space by a few percent while the spare disk is added to the volume. Adding the disk takes a bit of time, so this space stolen from snapshots will keep you running.
But don't forget to put the reserve back where it was when you're done.
One company in Silicon Valley, that uses filers very heavily (they store, for example, the source repository for their file server and Web-caching appliance code on filers, as well as many people's home directories, including their development trees), had a too-low snap reserve on one volume that holds home directories for many people in engineering; it would often fill up, and it would be well over the snap reserve when that happened, so the only way to buy back space was to delete snapshots, and at least one of the engineers there spent entirely more time than he'd like deleting snapshots to buy back space....
They subsequently added some more disks to that volume and cranked up the snapshot reserve, which should help, although, given that the volume is at 117% of the snap reserve, they may have to crank it up some more.