Rsvp?

In this situation it sounds kind of like an open sort of party so it's probably OK to call and ask if you can bring more people.

If you do recieve an official invite for just you or "(your name) and Guest" with RSVP on the bottom, it's impolite to ask if you can bring others. In these cases there is often a financial burden on the host, like at a wedding where the host is forking over $100+ per plate or paying for an open bar, but there's also an intimacy type of issue.