When you are creating a new campaign and get to the prospects stage, if you wish to assign existing prospects who aren’t assigned to any other campaigns to the current campaign, follow the steps below
Step 1: Click on Assign from existing prospects.
Step 2: Select from the list of prospects that you wish to assign to the current campaign and click on Assign.
Note: You could see only the prospects who haven’t been assigned to any other campaigns (since a prospect can be in only one campaign at a time)