EU, US, and Canadian citizens do not need a visa to enter Romania. For other cases, check this page of the Romanian Foreign Ministry. Note that Romania is in the Schengen area, so visas from another Schengen country, allowing multiple entries, are also valid.
You may also contact the Romanian Consulate in your home country. In case you need a visa, start procedures at least three months before the conference in order to be on the safe side.Â