Adding suppliers can be done at the same time while adding new products to the inventory. There is no need to add suppliers first before adding products because the product form automatically creates suppliers that are new to the database. There might be other situations where you wish to add suppliers first and complete more details. In that case add suppliers directly through menu-item Inventory ⇒ New Supplier.
The supplier database is a simple database with only one contact name field per supplier. The supplier contacts do not integrate with your customer contact database. When you email blast to your entire contact database from the IMS your suppliers are not included.
The supplier database can be browsed and managed through Inventory ⇒ Suppliers. Suppliers can not be deleted from the database if they are linked to products in the inventory.