Hi and welcome to the InfoPath Dev forums.
In the InfoPath rich client you could easily do what you need using InfoPath User Roles. However, User Roles are not supported in the browser form.
There are some workarounds (creating a simple web service to query the user who opens the forms permissions in another list somewhere for example) but most of these workarounds are a lot of work.
Does your solution require that you use a browser form? Or can it be a client form? User Roles in the client form work great and do exactly what you need without very much work at all.
Ted