God will come looking for you if you are not where He wants you. He knows where you are. We must prepare our hearts and minds to be of service to God. When the Spirit falls, it won't matter where you are... God will find you. These two men were prophesying within the camp and messengers reported it to Moses. Joshua was at that time Moses' assistant. He was apparantly disturbed at the disorder or rather the violation of what he perceived as the "chain of command" in the church. The anointing didn't flow down "Aaron's Beard" or something like that. (For an explanation of this allusion to Aaron's Beard see an earlier blog titled, The Brass Ceiling.)
We don't know why these two men did not go to the ordination service with the other seventy and Moses. But, the fact that God wanted them anointed is obvious. Moses answered to Joshua in Numbers 11:29 "would God that all the LORD'S people were prophets, and that the LORD would put his spirit upon them!" When God calls, it doesn't matter how you are ranked. That is grace!