I'm here to lend you a hand with your query. You can invite your assistant as a company admin or a standard user to your QuickBooks Online (QBO) company. This way, they can access your company with their login information.
But first, they'll need to create QuickBooks account. You can forward this link: Intuit Sign Up page.
Once done, have them provide you the email address they used to register their Intuit account. You can then invite them to your QBO company.
Click the Gear icon.
Select Manage Users.
Tap the Add user button.
Choose either Company admin or Standard user, then press Next.
Type in the requested information on the fields provided.
When done, click Save.
At this point, let your assistant know they can accept the invitation via the email address they used to register their Intuit account.
Learn more of the different user types in QBO in this article: User types in QuickBooks Online. This includes a short explanation of what each type can do. As well as the different access levels you can provide to your users.
Need further assistance? Place a comment below, and I'll be sure to get back to you.