Venture capital firm Hopkins Capital Group will acquire a passive 51% stake in Marco Consulting Group, executives from both firms said today. Terms of the deal werent disclosed. Frank E. ODonnell, Hopkins chairman and CEO, said the investment by wholly owned subsidiary Hopkins Asset Advisors in Marco, the leading consultant to union plans, could be followed by investments in firms advising other segments of the market. Jack Marco, chairman of Marco Consulting Group, said the deal, likely to be completed in the second quarter, calls for both him and Marco Vice Chairman Thomas A. Mitchell Sr. to remain at the firm for five years. Under the deal, ownership in the firm by executives and consultants other than Messrs. Marco and Mitchell will roughly double to 49% during the five years.