"engage voluntarily" is correct and usable in written English.
You can use it to describe something a person or group does without being compelled or forced. For example, "The employees volunteered to work extra hours to support their company's mission."
"The Peterborough project also works with short sentenced prisoners who engage voluntarily with services on release.
We engage voluntarily, because we are moved to do so.
This is how they sustain their position that the players are students who -- only secondarily -- engage voluntarily in a sporting activity.
Previous experience of dealing with the organisation may have led to reluctance to engage voluntarily with it, particularly if that experience was negative (e.g. refusal of training practice accreditation or a poor GP appraisal encounter).
The "logic" of these countervailing arguments, says an official, leaves you with a proposition that is vintage Brussels: "all countries must be engaged voluntarily".
"We viewed Everett as an attractive option and engaged voluntarily in talks with union officials to see if we could make the business case work.
A risky behavior you engage in voluntarily and that seems to afford plenty of benefit — think sun-tanning for that "nice, healthy glow" — feels less dangerous.
This study allowed younger (age 18 35, mean age = 19.9 years) and older drivers (age 62 81, mean age = 70.4 years) to freely decide when and how to engage in voluntarily chosen non-driving-related activities during simulated driving with conditional automation.
In part that's because those in Los Angeles County have developed a way to engage people voluntarily in in-depth services that go well beyond therapy alone.
Globally accepted standards of medical ethics – set forth by the World Medical Association in 1975 and 2006 – categorically forbid doctors from force-feeding competent prisoners who voluntarily engage in a hunger strike.
Through parametric analyses, we determine the net value that can be recovered from a returned product as a key driver for the firm to voluntarily engage in collection.
