Let's say there's an LLC company called Rainbow Panda that has a website called rainbowpanda.com

The LLC has two owners or partners if you will. They have a very comprehensive business agreement.

The rainbowpanda.com registrant information has the company name 'Spotted Lions' (a previous name that they had kicked around before they decided to create the business with a different name.)

Now I'm assuming if the business agreement was signed or the business license was granted to Rainbow Panda LLC prior to the domain name being registered it really doesn't matter who the registrant is on the domain name. They've got rights to it because it is their domain name and was registered by one of the partners of the LLC.

But what if one of the partners registered the rainbowpanda.com domain name, BEFORE the Rainbow Panda business was created. Does that part have sole ownership of the domain name and can do with it what they want? If the sold their part of the business could they refuse to include the domain name as part of the sale?

Sounds like a question for an attorney. Part of what's going to matter is what each member brought to the LLC in terms value. If the domain is not explicitly covered by the LLC documents, it's obviously not black-and-white. But there may be implicit rights granted by virtue of the LLC formation documents. Your attorney will know your state's laws and the implications therefrom.

it really doesn't matter who the registrant is on the domain name.

As far as the registrar is concerned, it does matter who's the registrant of the domain name. To them, the registrant is the one whom they directly contracted with, regardless of what happened behind the scenes of the registrant.

As Domainitor said, it's not necessarily black and white if the devil in the details aren't completely covered in any and all agreements among the parties involved other than the registrar. That's the part where a licensed attorney with real-world experience in these things come in handy.

The sooner this is addressed, possibly the better. I can't tell you how many ownership disputes I've had the (dis)pleasure of dealing with, especially when the parties involved insist on us mediating their quarrel.

Needless to say, registrars aren't necessarily judges in this sort of thing. Nor do they care to know the legalities of a gazillion legal jurisdictions all over the planet.