Guessing nonce - nonce pools/domains
The nonce value is indeed "guessed" by the miner. 

The miner can also choose a part of the coinbase transaction (what the stratum protocol calls the ExtraNonce2). Another part of the coinbase transaction is imposed by the pool: this is the ExtraNonce1, in terms of the stratum protocol.

Pools are supposed to assign distinct ExtraNonce1 to all miners, to prevent duplication of work.