You can see the "out of" point with the "great crowd." They are "out of" (Greek particle ek) "every nation and tribe and people and tongue." It is obvious that the writer means that the great crowd is a sub-group from among those larger groups. (Many commentaries also see that description of the great crowd as an allusion to the Abrahamic covenant.)
Similarly, the 144,000 are "out of" (same Greek particle ek) all the tribes of Israel. And each sub group of 12,000 are "out of" each tribe. This whole group of 144,000 are a sub group of a larger group described as,"all the tribes of the sons of Israel."
As was pointed out, the great crowd is seen working in the "sanctuary" (Greek naos) where it would be forbidden for people outside of the covenant to be.
They've made themselves "white in the blood of the Lamb." This relates them to being in the New Covenant.
Rev 7:16-17 is a quotation from Isa 49:9-10 which describes God's people being restored to their covenant heritage.
Everything about the passage speaks of the great crowd being in a covenant relationship with God. The fact that WT commentary on this passage disregards all that can only mean that their commentary is nothing but a self-serving lie.
Incidentally, see this post to see how the two groups in Rev 7 relate to seals 5 & 6 in Rev 6