There is no specific function designed link customers other than setting one as the master and linking the others. This is intended to create a single AR for all the associated customers such as one may do for a company with branch offices that are all paid for through head office.
That said, to achieve the result you are seeking, there are several ways to proceed depending on how you are currently populating your data.
The first suggestion is to assign all the customers with the same value in the "Acct #" field (General tab, lower right-hand corner) and ignore the warning given when setting the same value to multiple customers. On the Customer Selection window, you will see this value displayed in the "External Account" column. When you see a value in the column, enter that value in the "Search For" field and switch the "Index To Use" to be set as "External Account".
The second suggestion is to choose any field that is not in use (e.g., Category) and assign the same value to each member of the group. Now you can filter on that field to get the members.
Finally, as a variation of the second suggestion, there are two buttons on the Customer Selection window, Notes and Image, if these are not in use, you can put the members of the group in using these fields and have a visual indication similar to what you see via the "Related Codes" button.