There are 3 different ways to link insureds to the same policy. The method you should choose is based on the specifics of your case/situation. To link insureds to the same policy, follow one of these three methods:
If you have a master policy for a large number of insureds that are members of a professional association, then there are other ways. You probably have to clone the policy for each insured (this will allow you to pick different effective dates as the members join). The main policy will carry the premium and the clones are just there for COI purposes.
Lastly, if it is a Professional Liability policy with multiple insureds (like doctors or lawyers), then add them in the list of "Professional Liability" members (under "Insured Items" of the main named insured (the business entity)).