A business group is using an online survey to find out the extent and size of small home-based businesses. Hidden-Tech, a business networking group for ultra-small companies, is trying to gather results from 500 such firms in the survey. The survey asks the operators of such home- or small-office-based businesses to describe their business and the skills of the owner or workers. The survey also includes questions about how much money the companies spend in the region, and where their customers are located. "This has all been hidden up to now," said Amy Zuckerman, who owns a home-based marketing company and is the founder of Hidden-Tech. The respondents will be included in a directory of services operated by the group that could help companies sell their services, Zuckerman said. No information about finances and customers will be available to the public, Hidden-Tech said. General information about the contribution of such companies to the region will be released as part of a report on the survey to be released in the spring. The survey is available online at www.hidden-tech.net. Respondents must register at the site and then follow instructions after receiving a password, officials said. The costs of the survey are being paid by sponsor Western Massachusetts Electric Co. HiddenTech, an affiliate of the Regional Technology Corp., seeks to unite people in the arts, technology, financial services and other fields who have small business of one or two people. The group also includes some employees who work for larger companies from home.