When registering an account, I believe it asks if you want to be a delegate. You don't need to sign up as one now, even if you want to do it in the future. You can always "upgrade" an account to be a delegate later.
Delegates are to bitshares what miners are to bitcoin, except the use tremendously less electricity and are elected by the bitshares holders.
EDIT: and delegates have economic responsibilities to the network and ecosystem beyond simply providing block verification. ;-) thanks stan