What is the difference between the AAS-African Academy of Sciences and NASAC?
AAS is a regional academy with a membership of individual scientists, while NASAC is a consortium of science academies in Africa and as such, the academies are its members, not individual scientists.

How can a national academy become a member of NASAC?
By writing of its intention to become a member to the NASAC secretariat and providing a copy of its Statutes or Constitution or Memorandum of Association.