Can a client share legal advice from a lawyer?
Yes. In fact, by default a client is entitled to waive his attorney-client privilege. That means that the client may disclose the entire communications between the client and the lawyer rather than just the legal advice. It would be make no sense to outlaw the disclosure of lawyer's advice but not the full disclosure that the client's waiver enables.
How can Alice share this information with Bob and Chris without practicing law herself?
In any way short of misrepresenting, explicitly or otherwise, to Bob and Chris that Alice is licensed to practice law. In the absence of akin misrepresentations, Bob and Chris are not entitled to presume they have an attorney-client relation with Alice.
If the lawyer is wrong, and XYZing is actually illegal, is Alice responsible for Bob's and Chris's XYZing? Can they sue her?
It depends. First, it is unclear that the parties engaged in "XYZing" and whether it had any legal consequences, such as prosecution or losses.
Second, the nature of the relation between the parties determines whether Alice had a sort of fiduciary duty toward Bob and Chris.
Third, Bob and Chris need to satisfy the element of reasonable reliance even if Alice were their lawyer. Case in point: Someone's lawyer can advise to literally jump from a skyscraper to the concrete, but no reasonable person would rely on that advice.
