The main idea is to use 2 of 3 Multi-Sig. So the transaction is valid only if it was signed by two or three users.
The sending and receiving procedure is done internally on the blockchain and through Waves Web API.
Here you can find more information about Escrow in General:
(http://www.jbonneau.com/doc/GBGN17-FC-physical_escrow.pdf)
(https://en.bitcoin.it/wiki/Contracts#Example_2:_Escrow_and_dispute_mediation)
And here you can find our payment API documentation:
(https://docs.wavesplatform.com/en/development-and-api/client-api/payments-api.html)