Hi, grieveduncan-goo.
We can only invite users such as accountant in QuickBooks Online. We're unable to invite them in QuickBooks Self-Employed (QBSE). You'll want to move over to QuickBooks Online and then invite your accountant after. Or you can give your accountant your QBSE login credentials.
If you opt to move from QBSE to QBO, you can use this link for the detailed process: Switch from QuickBooks Self-Employed to QuickBooks Online.
Here's the QuickBooks Self-Employed Overview page that provides more info on what we can do in your QBSE account.
If you need more assistance concerning your QuickBooks, please let me know by leaving a comment below. I'll be sure to get back to you.