The Federation of Small Businesses (FSB) was giving evidence to the Northern Ireland affairs committee.
Voters will go to the polls on 23 June to decide whether the UK should remain a member of the EU.
The FSB's Roger Pollen said its members are "dividing roughly into two substantial camps" for leave or remain.
But he added that a substantial part of them need more information before they can decide on facts rather than instinct.
Mr Pollen said there was uncertainty about what leaving the EU would mean for businesses.
"Looking to remain you're dealing with a largely known quantity, looking to leave there is uncertainty," he said.
He used subsidies for farmers as an example, saying "there's no guarantee" that agriculture in Northern Ireland would be as high a priority for the government as it is under the current settlement.
The Democratic Unionist Party MP Gavin Robinson suggested farmers could get a more generous subsidy outside the EU.
FSB member Patricia O'Hagan, who runs an IT firm in Belfast, said she was concerned about how leaving the EU could impact on her ability to compete for government contracts in EU countries.
She also asked how it would affect her ability to employ people from EU countries.
"We have employed people from Poland, Lithuanian and other countries - if that became more difficult I would have concerns about being able to get the skills we need," she added.
Mr Pollen said one member of the organisation who wants to leave the EU argues that it would be easier to give government support to businesses as EU state aid rules would no longer apply.
That member said a major impact would be that corporation tax in Northern Ireland could be reduced without having to cut the block grant that Northern Ireland receives from Westminister.
Businesses need more information before they can reach firm conclusions on the European Union referendum, a Westminster committee has been told.