@HJoci30
No, no, you don't have to go in individually at all. Damage "overlap" only exists at phase change. At the beginning of the raid, if 5 players attack at the same time, and each does 1M damage, after all 5 are finished, the raid will be 5M damage lower.
However, close to the end of a phase is where you want to be careful. If Phase 1 only has 1M damage remaining, and 5 players go in simultaneously and each finishes the phase (i.e. each does 1M damage) - phase 1 will end, but 0 damage will carry over to phase 2.
Each of those players would still see their score increased by 1M damage, but the guild would have effectively "wasted" 4M damage because those extra players don't actually deduct from the raid boss health.
All raids work the same way in this regard.