This sort of takes idea’s from all sorts of diff posts from others and tries to find a mid-way thats more reasonable (at least I think so anyways). Thats my disclaimer.
What if the levels you could enter based on group were Tier’d instead of singular?
ie. If you are in the first tier (1-9) you can enter any 1-9 group and progress your personal level by 1 on each completion.
So.
Group of 1 3 5 9 9 enter Level 9 and complete.
now it’s 2 4 6 10 10 afterwards.
This group would then could do difficulty 9 over again to catch other people up or replace the two 10’s who are in a new tier and continue at whatever highest level their group has.