Jehovah's Witnesses believe that many will be saved who are not now Jehovah's Witnesses.
Jehovah's Witnesses would have no reason to engage in their preaching work if they did not believe that there wasn't any hope for others. They are certain that God's mercy will extend to many kinds of people. At Acts 24:15, the apostle Paul stated that God is going to resurrect even "the unrighteous."
Notice this quote from the 2000 publication, Jehovah’s Witnesses—Who Are They? What Do They Believe?, Questions Often Asked by Interested People, as found at the Official Website of Jehovah's Witnesses:
"Do you believe that you are the only ones who will be saved?
"No. Millions that have lived in centuries past and who were not Jehovah’s Witnesses will come back in a resurrection and have an opportunity for life. Many now living may yet take a stand for truth and righteousness before the “great tribulation,” and they will gain salvation. Moreover, Jesus said that we should not be judging one another. We look at the outward appearance; God looks at the heart. He sees accurately and judges mercifully. He has committed judgment into Jesus’ hands, not ours.—Matthew 7:1-5; 24:21; 25:31."
(Additional information can be found in the 08 11/1 Watchtower article p. 28 entitled: Do Jehovah’s Witnesses Believe That They Are the Only Ones Who Will Be Saved?)