To import clients using an Excel sheet we must access:
Tools -> Importers -> Import customers.
Here are the options to import Excel clients:
*Important: the doc. must be in CSV format
As we can see, here we must select the fields that we want to process and we must indicate their position in the Excel file that we are going to import. If the surname and first name are in the same field we must select the option: "Take surnames from the same field".
In case the Group field to which the clients belong is not in the file, we should go to the dropdown that says Indicate group to assign the clients and indicate the group to which they belong:
We also have the option to indicate the sport:
To import the Excel file in CSV format we must select the file by pressing the icon 
Once the file is selected we can click preview to see how the file will be imported, see if the fields are correct etc. This way we can modify some field before importing directly.