These two theories both agree that interest groups have the ability to participate in the American political system. However, that is the extent of their similarity. They disagree on the degree to which interest groups of all types have the ability to participate on an equal footing.
Pluralists believe that interest groups can all participate on a level playing field. They argue that those interest groups who have the largest memberships and are most motivated with regard to a particular policy issue will win on that issue. The system, in this view, is fair because it allows all groups to participate and it allows those groups that care the most to win.
Elitists argue that the playing field is not level. They argue that there are some interest groups that have an inherent advantage. These are the interest groups that have the most money. These groups can typically use “insider” tactics like lobbying to defeat other interest groups even if those groups have more members and more motivation. In this view, power can essentially be bought and the American system is not nearly as fair and open as the pluralists say it is.