Hi @Benben2 ,
Do you mean that you add a Landing screen in your app, where you need to type username and password?
Do you use the LookUp function to validate the entered username and password with that from your Excel table?
Further, have you shared your Excel file resource to these end users?
If you store these username and password in a Excel file, you should also share this Excel file to these end users, then these end users could access the Excel table resource from your shared app.
Actually, Power Apps canvas app inherits data access permission from data source itself. If you want these end users to use your data source in this shared app, you should also share your data source to them.
More details about sharing canvas app resources in Power Apps
Also please make sure these end users have entered proper username and password (which has been recorded in your Excel table already) within the Landing screen.
Regards,