When wishing to set up account customers, you must firstly make a customer group for account customers. This is done under Customers > Group Browser > Add Customer Group. 

Firstly; you will need to give the group a name so you know which group to add customers too. You can also choose to give a particular group a discount rate which is marked as a percentage off. By setting the group as a 'Default Group', 'Yes', all customers will automatically be assigned to this group.  Please note; this means all customers signing up through the checkout

By selecting 'Account Customers'  'Yes', a contents box will appear so you can add your message for your account customers to see at the end of the order process.

To add a customer to a group, simply go into their details and select the group from the drop down menu under 'Customer Group', as shown below. 

Once the customer has been added to the group, they will be able to shop online and receive any discounts you have applied to them. When the customer goes to process their order, they will not pay there and then. The message you created for the group will appear instead with any information you wish to give the customer as shown below. 

Note: Customers can also be added to a group via the customer browser tick box & footer selection.