What is the best way to add multiple locations for a single company?
You will need to create a separate company profile for each location. You cannot have more than one company with the same name.
For example, PipelineDeals has an office in Philadelphia and Seattle. One profile could be 'PipelineDeals Philadelphia' and the other would be 'PipelineDeals Seattle'. Then if you would like to associate these two companies together, you can do that by adding a Multiple Association Company custom field.