As mentioned, it is common to share seats at a table in a hawker centre. If you’re at a table with vacant seats, at some point you will definitely be asked, ‘Is this seat available?’
At times like this, reply with ‘Sure! Go ahead.’ or if you would like to try your hand at colloquial Singaporean English (otherwise known affectionately as Singlish), try saying ‘Can! Can sit! Okay (lah)!’.