A good discussion of membership and attendance data in Data pointing toward reality upstream. Kirk Hadaway, who made the report to Executive Council, answers this question in the comments by Tobias Haller:
Can anyone provide a figure for overall church membership and attendance across all churches. It then might make sense to see what the TEC "share" of that is, and if that share has changed. The raw stats seem to me to be singularly uninformative in addressing the problem of decline, if the decline is due to a wider movement in the public rather than to something we are doing or failing to do
No, there is no meaningful figure for overall membership and attendance. There is no one who is able to collect figures from all denominations (and independent churches) and the statistics for many large denominations are useless for computing totals. The Roman Catholics, for instance, basically count all baptized Catholics and do not count attendance except in certain dioceses. The Church of God in Christ (a large Black denomination) has not reported in many years and has what are probably grossly inflated numbers. The same is true for several other very large historic Black Baptist groups.
So if you can't count members and attendees directly, what about surveys? Americans presume membership, so that figure is unreliable. Attendance is exaggerated by nearly 100%, as I have shown in several published articles.
The best you can do is trace membership in the denominations that have reliable figures. The long-term pattern was mainline decline and conservative growth, with the overall total remaining about the same--until the last 5 years when the overall total began to decline. Mainline decline got worse and several large conservative bodies began to decline (Lutheran Church, Missouri Synod and Southern Baptist Convention, and a few others). If you want to add Roman Catholics to the mix, they have declined severely in many areas, but to a certain extent immigration has enabled them to keep up membership, if not attendance. Mormons have grown, but not nearly as much as reported. They do not drop people from their membership rolls and they have a very high birth rate (the rate of natural increase in Utah is higher than Mexico).