If you’re in a multi user access, the assistants you added should be on the same shared network so they can access the company files.
To set up and use QuickBooks Desktop multi user please check out this article: Set up a multi-user network. Follow steps 2-7.
Resolution for 'company files issue: 'New QB User on a Different Computer log in using QB Desktop' available: Yes.
The company files issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the company files issue yourself by looking for a resolution described in the article. If it is a complex company files issue or you are unable to solve the issue, you may contact us by clicking here or by using other company files support options.
For more information about resolving issues related to company files in Quickbooks/QBO or to get support for the issue 'New QB User on a Different Computer log in using QB Desktop', click the link below: