"Summarize and assess Domhoff’s central argument."

G. William Domhoff believes that social class membership in the following institutions: education, social circles, marriage, and volunteer groups (which play an important part in an area where complete government control "[could lead] to communism") [pp.280], provide the middle/upper class and upper class with an unbalanced share of power in the world. Domhoff claims many Americans deny that social classes are based on wealth, yet they harbor a fascination with rags-to-riches stories and "the trappings" of wealth [pp.272].
I agree with Domhoff and his idea about wealth and power in the world. Isn't the common saying, "money equals power"? We see it with celebrities like O.J. Simpson being acquitted for murder when it was obvious (to me, anyway) that he was guilty. We saw that money made it possible for Sean "Puffy" Combs to walk away from the charges against him. If the wealth and fame did not influence the decision of the judge, I believe it was quite possibly the standing of the high-paid and well known legal representation that played a role in the case.

"Do you think Domhoff provides sufficient evidence for his argument, or not?"

Domhoff provided evidence from numerous sources. He cited studies from sociologists and authors (including Randall Collins, Digby Baltzell, Susan Ostrander, Erving Goffman, Steven Levine, Gay Pauley, and more) and sociological studies (which include Gary Tampkins, the author's own personal research, Beth Ghiloni, and more). Domhoff used various sources and explored each of his arguments extensively. The ideas were supported with statistics from case studies, biographical directories, surveys, and interviews. Some areas of Domhoff's article were more convincing than others. [see below.]

"Why or why not?"

Domhoff's section on education was well supported with facts. He reports that many upper-class students never see the inside of a public school. This statement is supported with the statistic "only 1 percent of American teenagers attend independent private high schools of an upper-class nature [pp.273]." Sociologist Baltzell reiterates how schools " [create] an upper-class subculture on almost a national scale in America [pp.273]." Students attending private schools create a common link with other people of a high social standing. Therefor, they have connections all over the country if need be. Private schools also churn out more students entering the fields of business, finance, and corporate law.
According to Domhoff and his sources, social clubs play a major role in the lives of the upper-class. Each club has membership fees and dues along with interviews and a review process. Although clubs are seen as prestigious and exclusive (which they are), it seems that many are members out of feeling of obligation and tradition which honor past generations. Domhoff explains how "social clubs are not a place of power" but instead a place of "bonding [pp.277]." This statement seems to go against Domhoff's original statement that these social groupings lend to power in the world. In this area, Domhoff did not convince me that these social clubs lend to political or societal power.
Domhoff's section on the debutante tradition did not lend to his original argument, either. Many of the interviews in this section (interviews made up much of the "evidence" here) portrayed a sense of obligation and tradition. While the young women wished to impress and please the older generations, may thought the debutante ceremony was old-fashioned and passe. To me, this rite of passage seems overdone and flashy.
Women in the upper-class volunteer because of a sense of obligation to community. Women claimed to be involved in volunteer efforts out of a desire to keep government at bay [pp.280]. Women are subordinate to men, yet exercise power through their volunteer work. While I understand how volunteer efforts keep the government out of certain areas, I do not see how volunteer work makes the upper-class group more powerful than any other. Domhoff's examples and evidence did not convince me here.
And in marriage, Domhoff did not convince me, either. It appears members of the upper-class do not marry exclusively within their class. Although many members of the upper-class tend to stay there even after marriage, I believe since marriages weave old and new members [pp.283], lower-class members of society are given the chance for money and power. Marriage appears to be a major chance for those of a lower income to move up in the ranks. And many people DO marry for money. The window is obviously open for opportunistic men and women to get into wealthy families.