Be very careful with these type of opportunities, especially if a lot of time has been wasted on stuff that adds very little value in terms of getting the company traction. If the company has customers already that's one thing. If the company has just some standard incorporation stuff, an idea, some failed prototypes, a bunch of sketches, etc, think long and hard before jumping on board because the other "original" founders will always hold this over your head and in one way or another you'd be second-class. A smaller percentage is one thing, but look out for any vesting or other provisions they try to bring up that may severely impact your leverage and position within the company.