The "great crowd" is a small subset of the "other sheep" It is only those "other sheep" who survive the great tribulation. The Revelation book actually states that they are the "firstfruits" of the "other sheep" (p. 203) Those of the "great crowd" who who die prior to Armageddon lose their status as "prospective members" of the "great crowd" and are relagated to the much larger group of "other sheep" who are resurrected. JW literature doesn't deal with this often, but when it does, it is very clear:
"All those making up the "great crowd" of Revelation 7:9 are "other sheep," but not all of the "other sheep" of John 10:16 constitute a part of the "great crowd." These terms are not wholly synonymous." (The Watchtower September 1 1966 pp. 543,544)
"Now, what can be said about the identity of the "great crowd" mentioned at Revelation 7:9? Well, look at verse 13 and the question, "Who are they and where did they come from?" We find the answer at Revelation 7:14: "These are the ones that come out of the great tribulation." So the "great crowd" is composed of those who come out of, or survive, the great tribulation. As verse 17 says, they will be ‘guided to fountains of waters of life’ on earth.
Understandably, though, for these to survive the approaching great tribulation, they must earlier have washed their robes in the blood of the Lamb, becoming true worshipers. Hence, though Revelation 7:9 is describing this crowd after the tribulation, we may apply the term "great crowd" to all with earthly hopes who are rendering Jehovah sacred service now, just before the great tribulation breaks out with the nations’ attack on false religion.
In summary, we might remember "other sheep" as the broader term, encompassing all of God’s servants having the hope of living forever on earth. It includes the more limited category of sheeplike ones today who are being gathered as a "great crowd" with the hope of living right through the impending great tribulation. Most of those loyal Christians alive today are of the "other sheep," and they are part of the "great crowd" as well." (The Watchtower April 15, 1995 p. 31)
http://www.jehovahs-witness.net/jw/friends/205271/2/When-did-the-other-sheep-class-begin-to-form