Where is the next uid from adduser pulled from?

[Francisco Reyes]

I have scripts to add new users. However, after that any port that installs 
a user creates it with a UID after the ones I made.

For example I want all employees to have uids starting at 5000, but I would 
like too port installed uids to be 2000 and up. After I add some users (ie 
say last user is 5008), the next port that installs a user and doesn't 
specify uid.. then will get 5009.

Tried looking for the adduser program, but could not find adduser.c

Any pointers would be greatly appreciated.