A campaign should always be targetted to group of users or multiple user groups. So the list of user who are targetted for campaign can participate in campaign and through this way they are registered to campaign and this information will be used to report the success rate of campaign and other reporting purspose (also these users does have more relative information than to normal registered user because targetting can be done based on those properties). Now apart from those users there are other users in system which has been created for some other purpose based on application designed like registered users/new letter group users etc... and not necessarily belongs to campaign. So its other way around thats why those users who are registered through campaign can be seen as part of users/groups but those who are created as separately through other process can not be seen as part of campaign leads/list.
I hope above helps to clarify your doubt.
Please refer this too : http://dev.day.com/docs/en/cq/current/wcm/campaigns.html
Thanks for the clarification Pawan.
So according to this, If we have some users in the system (not added as campaign leads) and If we want to add them to a campagin as leads (so we can target them for our campaign), We need to import or add them again using campaign leads menu ?
Is that correct ?