Nearly 300 companies and business groups across the country, including at least 30 from Massachusetts, told the US Supreme Court on Wednesday that the Defense of Marriage Act requires them to discriminate against some employees.

A friend of the court brief, filed Wednesday in a case to be heard by the justices next month, was joined by many large employers across the state, including EMC Corp. and State Street Corp., Akamai Technologies Inc. and Blue Cross Blue Shield of Massachusetts, Inc.