Hello G R Taylor Accountants, 

Welcome to the Community page, 

If you have Quickbooks Online to add a new client on to one of your licences, if you click on clients>click on a available placeholder>fill in the information in on the placeholder (so name of the clients business and that)> then go into that Quickbooks account placeholder and invite user (client) to join the account so they can use is as well. 

You can click the QB logo beside the name in the clients list to access the clients account or by going to go to quickbooks- and can click on the client there as well. 

You can have a look at this link here as well that tells you some information about adding a client.