Human-resources startup Zenefits Inc. is falling short of its aggressive revenue targets and has started to curb expenses, making it the latest highly valued venture-backed company struggling to meet investor expectations.

Since late summer, Zenefits has frozen hiring in certain departments as sales teams have repeatedly missed targets, according to people familiar with the matter. It has cut the pay of some employees and dozens of people, including at least eight executives, have left or been fired, the people said.

Zenefits...