I have removed the other Club Leader (Zoey) from the my Household and had Rebecca start a Club while part of the Art Lovers Club and the goal also completed. I also tried the Pancakes Household with Eliza by having her join Art Lovers and then starting a Club, she also got the Goal as well. It appears that even leaders with 8 Sim Clubs part of different households will still complete the Lead a Club with 8 Sims goal.
I used testingcheats true and aspirations.complete_current_milestone to get to the Leader of the Pack milestone for the test which was used on Urblake, Rebecca, Zoey, and Eliza from the Pancakes household. I started Art Lovers with 6 Sims and set the Club Activity to Paint to allow me to get 750 Club Points to get both Capacity upgrades. To prevent the Welcome Wagon from appearing during test, I edited the Municipal Muses lot in Willow Creek to include a basement and easels and did the Gathering on that lot.
I could guess why none of the default Clubs have 8 Sims because belonging to one the Clubs while being a Leader of another Club would allow the Goal to complete instantly upon reaching the 4th Milestone.
Thanks for testing. Adding this. The text definitely suggests that you need to be the leader of a group with 8 Sims and not leader of a group and in club with 8 Sims.
