This is a very hard question to answer precisely… it depends largely on how you define Al Qaeda membership or affiliation. The roots of Al Qaeda date back to the original jihadists that fought the soviets in Afghanistan in the 80’s. Many of the fighters that had personal relationships with Usama bin Laden during this conflict, became key members of the Al Qaeda organization.
As the organization grew and became more popular, various other organizations aligned themselves with Al Qaeda- to varying degrees. The following is a list of some of the better known muslim extremist organizations that have merged or aligned themselves with Al Qaeda.
Ayman al Zawahiri, formerly the leader of the Islamic Egyptian Jihad, merged his organization with Al Qaeda in 2001. (This organization had been operating in Egypt since the 1970s prior to their affiliation with Al Qaeda). For many years now, Zawahiri has been the #2 man behind bin Laden.
In September 2006, the Salafist Group for Preaching and Fighting (GSPC) also merged with Al Qaeda. They now identify themseles as Al Qaeda in the Land of the Magreb.
In November 2007, Abu Layth al Libi and Zawahiri publicised a combined statement saying that the Libyan Islamic Fighting Group and Al Qaeda were merging.
The most significant one, however, would be Abu Musab al Zarqawi proclaiming his allegiance to bin Laden and Al Qaeda in October 2004. His organization was known as Tanzim Qaidat al-Jihad fi Bilad al-Rafidayn (The Organization for Jihad in the Land of the Two Rivers (Mesopotamia)). Following the merger, it is known simply as Al Qaeda in Iraq.
The previous poster is really dead on, the core Al Qaeda membership is actually pretty small- probably a few hundred. These are individuals that have personal relationships with the inner circle of Al Qaeda leadership (Usama bin Laden, Zawahiri, Abu Ayub al Masri etc.). The individuals and groups that maintain communication and take guidance from the leadership are definitely Al Qaeda members. However, many more groups may share a similar ideology or goals to Al Qaeda, though they lack direct relationships. The number of people that fit into these types of groups would be a significantly higher number- likely several thousand, or tens of thousands.
Additionally, the number of people that identify with Al Qaeda has risen and fallen drastically in the past 5 years. Following abuses by the US military in Iraq, large numbers of individuals from the muslim world came to fight Americans and to align themselves with Al Qaeda. In the past 12–24 months, the organization has been significantly reduced. This is somewhat due to members being killed or captured. More often, however, individuals and groups have left Al Qaeda after becoming disenchanted and dissapointed with the oganization. A number of individuals chose to leave Al Qaeda after being dissapointed with some of the techniques the group is willing to use to achieve its goals.
That is my two cents on the question… I know it’s a bit of a round about answer, but there are likely a few hundred genuine Al Qaeda members in the world right now. Several thousand more individuals are part of groups loosely aligned with Al Qaeda or that share very similar goals and ideology.