[TYPO3] frontend users login

[Sergio Catalá Gil]

Hello list, 

I've created some PHP code for allowing a user from a certain database
to connect to the frontend of TYPO3. 

I check its username and its password, and if everything goes well (and
here's where I'm getting lost) I would like to create its user and group
in TYPO3 tables.  

Does anyone know where to find some PHP code to do that?

I've been trying to find without success how users are logged in the
frontend through the default plugin for logging. 

Thanks, 
Sergio