Why are tribulation believers grouped with Old Testament saints rather than the church?
Tribulation believers are grouped with Old Testament saints because they are not members of the body of Christ and cannot be. The tribulation is the time of God dealing with Israel, with the temple operating, and it is primarily a Jewish period. They are therefore separate from the raptured church.
Les explains that after the church is raptured and is in the Lord's presence in a resurrected state, the horrors of the seven-year tribulation unfold. During that time multitudes believe and are martyred, both Jew and Gentile. They are not members of the body of Christ, not because they missed the rapture, but because they simply will not be included in the body; they cannot be. Since the tribulation is the time of God dealing with Israel, with the temple operating, and is primarily a Jewish period, Les places these tribulation believers back with the Old Testament believers in that particular resurrection group, including the extra 75 days beyond the tribulation when Daniel and his fellow believers stand in their lot.