To use a miniExtensions portal, you need to have a linked record field in your data table pointing to your users table. Some users may have other fields determining which potential user a record is related to, like single line text, single/multiple select, formula, email or collaborator. You can turn these fields (and others) into linked record fields for your portal quickly and easily. Here's how:
In your data table on Airtable, find the field that determines which records should be visible to which user.
Duplicate the field by right-clicking the field's header and clicking Duplicate field.
If shown, make sure Duplicate cells is enabled.
Right-click the header of the field's copy and select Edit field.
Change the field type to Link to another record and select Create a new table when asked to pick the table to link to.
Set the Name for the new table to something that makes sense for your use case, e.g. Customers, Vendors etc.
Hit Save
You now have a linked record field that links each of your records to their respective record(s) in the users table!
If everything looks correct, it's recommended to remove the original field to avoid confusion in the future.
You can now return to miniExtensions, go to create a portal and select the newly created table as your portal's users table.