Depending on how you intend to use this, you have two options.
1. You can just add the business as an asset of the client, partner or whichever entity owns it.
Click on Assets, then Add an "Other" asset. You'll be able to enter a name valuation and valuation date.
This method makes sense if you're just recording it as an asset of the client and won't be advising the business.
2. If the business is an advice client, then you'll probably be better off creating a business entity. So click the Add Business Entity button as shown below:
You'll then be able to enter all the relevant details about the business. You'll be able to link individual clients to the business entity through the Related Personnel link.