I don't think an even split is expected (or even suggested -- consider that Sequoia takes around 30% only!), but a good rule of thumb is that, prefunding, someone who joins is a cofounder. It really changes your perspective when you're part of a team with a vision, maybe some duct-tape prototype, and the idea that, come hell or high water, they're in this together.

Bill Joy joined Sun after the founding. One of my partners joined us later, too, They are absolutely cofounders.

That said, the biggest red flag? Dishonesty. If you smell a whiff of it, I would advise serious caution.