Note: As stated in other question creating relations between tables/models doesn't require adding foreign keys
I'm not quite sure what do you need it for, but here's description of two most common cases (screenshot shows how to achieve each case with Extension Builder)
Creating relation to FrontendUser model / fe_user table
In TYPO3 ver. 6.2 fe_user has a model as well: \TYPO3\CMS\Extbase\Domain\Model\FrontendUser, you can just add this as common relation in Builder modeling tool (fig. 2 at screenshot) in the field \Fully\Qualified\Classname (you need to Show advanced fields to set it (fig. 1) use this FrontendUser and it will create proper SQL and TCA definitions.
Real extending FrontendUser (FU) model
On the other hand if your People* model just extends the fe_user table, because you want to reuse existing fe_user table for your people, you can also extend FrontendUser model for real (fig. 3) in that case you'll inherit all getters, setters from FU etc. and you won't need to write them yourself.
In such case your model will be a separate type of fe_user, the discriminator field is by default: tx_extbase_type and builder will add new type like Tx_YourExt_Employee
* note: For model names you should use singular form i.e Man instead of People
