“Lift-outs” are otherwise known as corporate raids and are hazardous to your legal health, especially for retained practitioners.
The search business isn’t passive anymore. The people you want are employed. Maybe not “happy” (whatever that means) but at least satisfied enough to heat their seat.
There’s only one practical way to find them: Calling them on the job; then interesting them in looking for another one. It’s usually legal, too. Unless you’re paid in advance to a: Call into a specific employer, and b: Raid an entire department or discipline.
That’s what the civil law calls a conspiracy. Even then, the employer needs to connect the calls with some other civil wrong like invasion of privacy, unfair competition, inducing breach of contract, interference with contractual relations, interference with prospective economic advantage, or fraud.
If it can connect them, you could be liable for compensatory (actual) damages by loss of the employees. Then unlimited punitive (to punish) and exemplary (to make an example) damages can be awarded at the discretion of the judge or jury.
This is one time when it pays you not to have a retainer. Even so, victim companies can make life miserable, and it’s better not to participate in direct “lift-outs.” Better to just go after the lead dog of the team and let them do the dirty work after they leave and join their new company. Getting compensated for this type of deal requires a wink and a nudge that can backfire against you later.