You need to have a probate attorney in the state where your grandfather died review the will and deed and any other relevant probate documents. Generally the will gives broad latitude to the executor of an estate but if he causes damages you the beneficiary under the will may be able to file a lawsuit. As this sort of litigation is expensive and frequently takes years (read Dickens novel Bleak House for an example) it is best to try to resolve the matter without the necessity of litigation.
So long as the property is part of the estate, the executor (referred to as a Personal Representative in Montana) probably has authority to make the kind of determination. However, those decisions must be reasonable. As a beneficiary, your mother could challenge the actions that the Personal Representative is taking. It would require court intervention and can be an arduous process.
Attorneys Sullivan and Bernick are correct. The Executor must act reasonably and, if he's allowing a bunch of folks to use the house rent-free, he's probably going beyond what a probate court would consider "reasonable." If your mother wants to get these people out of the house, her best bet is to retain her own attorney to push the Executor to act properly. If he does not do so, then the attorney will have no choice but to bring this matter to the attention of the probate court. Good luck to you and your mother.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.