The issue is well documented at https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Unable-to-add-users-to-a-BIM-360-account-that-has-BIM-360-Design-Collaboration.html?us_oa=akn-us&us_si=7f1b0201-da5b-4658-8365-1526338d172e&us_st=members%20cannot%20be%20added%20you%20have%20reached%20the%20maximum%20number

Another workaround you can try is to add members to the Project from the Project Admin module instead from Account Admin Module.