On Outlaw a Party represents one side of a deal, which may include multiple individual signers or entities, as shown in this example:   

Recall that when a Party element is filled in with signer information, this signer now appears in the Manage Users Panel, which opens at upper right of the contract:

Additional signers on this side of the deal (in this example, "Students") can be added here by clicking the "+ User" icon and entering information for these additional signers, making sure to select the correct Party from the first drop down menu under each entry:

Now your deal has multiple signers within a single Party!