Membership Catagories

I am trying to create a membership list for our chamber of commerce. I have created a members table and a catagories table. The problem is that some of the members have many catagories. Ex: ABC Advertising Co. does graphic design, photography, website disign, and weddings. I'm using the menu drop down to see the catagories in the table for the catagoryID which brings up the member just fine, but I can't figure out how to add the other catagories so they would be recognized by the drop down menu. I'm new to dataassist so just learning. I'm sure this is probably an easy task, but just don't know where to start. Any help would be greatly appreciated. I've read something about joining but just don't understand what to do at this point.
Hal...