In case you would like to add many trainees to a specific group, simply use the Import tool. Go to 'Trainees' and click on the 'Add trainees' button in the top-right corner. Click 'Import many trainees' from the drop-down menu to open the Import form.
Make sure to have a spreadsheet program (Excel or Google Spreadsheets) to create a file with all the trainees that you would like to import. Make sure that the first row in that file has the titles of the columns, being:
Only 'EmployeeNo' is optional, the other items are required. Also make sure that the titles in the first row match the names as mentioned here.
On each other row fill in the data of your trainees. Need an example sheet, please check here: https://drive.google.com/file/d/1vkBNwLIEWTcCi1JSIvxJFsiC8Oc7fD5l/view?usp=sharing
When you have your file ready, save this file as a Tab-Separated-Values' file. In Google Spreadsheets this will be saved as a TSV file. In Excel this will be saved as a TXT file.
Upload your file and select to which group(s) these trainees should be member of. Click 'Import' and wait until all trainees have been imported.