In parashat Masei (34:17), a list of the leaders responsible for the conquest of Israel is given. Each tribe has a leader except Reuven and Gad. Even Menashe who's home is also across the Jordan has a leader for the conquest.
The subject of the pesukim is the division of the land for the settlement of the tribes. Thus, the leaders mentioned are the representatives of the tribes who will actually own the land.
The names that are given are the leaders who "took possession" of the various territories on behalf of their tribes. Since Reuven and Gad did not take posession of any territory to the west of the Yarden, they are not listed. Since part of the tribe of Menashe did have territory in Eretz Yisrael, then the leader of the tribe was listed as taking possession on behalf of the tribe.
Yehoshua and Eliezer (the kohen gadol) are listed as representing the nation as a whole to preside over the division of the land. Once each tribal leader got the general territory owned by his tribe, he subdivided it among the families of that tribe.