The adventuring I'm preparing has the PC's offering some merchandise to a Hutt. During the negotiations, the PC's rival crew interrupts the negotiation, claiming that the merchandise belongs to them.

The rivals are using deception because they do have any rightful claims.

How would you play this out?

I'm thinking it's a social combat between the two crews, with each sides check opposed by the other with successes and failures being used to convince to the Hutt who he's going to pay. Despairs from both crews helping the Hutt to decide to just take the merchandise and throw them all out.

What do you guys thing?

How can the rest of the party get involved in such an encounter?

In a case where two parties are trying to convince the same person to take their side you would use a competitive check, as outlined on page 25 of EotE, with a difficulty set as though the check is opposed by the Hutt for both parties.

The best way to bring the whole party in on an action like that is to allow them to perform other actions to either add setbacks to the other side, or grant boosts to the person rolling the social check. You could also set up the encounter so that you keep score behind the scenes, and decide the encounter based on which faction scores 3 "points" first, one of which can be attained by arguing the case successfully, and others could be obtained by acquiring pieces of proof against the rival side by hacking or sneaking onto their ship, or by challenging one of their members to a fight for the Hutt's amusement, by acquiring some kind of gift for the Hutt, putting on a performance that entertains the palace guests, or any number of things that might score points with a Hutt.