Stigmergy occurs as a result of individuals interacting with and changing a landscape. For a Web based stigmergic system, the landscape is a Web site and visitors to the site are the individuals.
As you cannot expect visitors to write the code necessary to make changes to a Web site, there has to be agents - acting on their behalf - to do this for them.
Many different kinds of agents can be used, depending upon the sophistication of the Web site. The highlighted links on the left will give you a starting position for the design of suitable agents that can be used in stigmergic systems.