You may wish to link your contacts from another application (Siebel, SalesForce, other CRMs)
Basically the architecture steps are:
1)Creating an infrastructure in C# or PHP such that a link can be made /somesite/linktoyourmanagecampaign{Contact I}
2)Campaigns you care about can be stored in a configuration file XML with a structure similar to:
<CampaignDefinition>
<Title>Campaign Title that matches name in Eloqua</Title>
<Description>Some description of the campaign</Description>
<AddSharedList>Campaign Title - Participate</AddSharedList>
<DeleteSharedList>Campaign Title - Remove</DeleteSharedList>
</CampaignDefinition>
3) The link in #1 presents a list of campaigns, and allows add/delete of contact to the various shared lists
In this way anyone can add or remove contacts to a given campaign
4) Also , you might want to report on the web this sort of activity via another web page