Hi JuliesABCs.
Being able to connect to your clients QuickBooks is a great way to have access and make any changes to their books. I'd be happy to explain more about how users can be invited in QuickBooks.
In order to be added or invited to an account the user first needs to have a QuickBooks Online subscription. As your client has QuickBooks Self Employed, inviting an accountant or adding another user is not able to be done as self employed is a single user account. There is no Chart of Accounts and the majority of accounting has been streamlined. If your client wishes to invite you into their account they'll need to upgrade to QuickBooks Online which can be done by following the steps in this article.
If you have any questions let us know and we'd be happy to help.