The answer is essentially that you can't. You can invite them to a party (XBox) or open a chat (Games for Windows Live) or something similar. But there isn't a built-in chat client, sorry.
As I understand it business partnerships work a lot like Civil Unions (source). The goal is that a functional business relationship should have the same benefits as something like Marriage. Namely:
When you are a player form a partnership, it only takes effect when you are playing together.
While playing together, every purchase made (by either player) is split evenly. So if Player A buys a house worth 10000 gold. Both players pay 5,000 gold. If one player only has 1000 gold, they spend all of it, and the other player spend 9000. Same for items.
Income from properties, jobs, chests are split 50/50.
When dissolving a partnership (or marriage) all joint properties are sold and the proceeds are split 50/50. This happens whether or not you're in a game together.
Best Answer
This might help.
The answer is essentially that you can't. You can invite them to a party (XBox) or open a chat (Games for Windows Live) or something similar. But there isn't a built-in chat client, sorry.