Your members, from the spreadsheet you already have
Drop the club's member list on the Members page: every row is checked against the club before a single member is created.
The app in use, recorded.
Every club has its member list somewhere, usually in a spreadsheet. On the Members page, the upload button takes it as it is — .xlsx, .xls or .csv — or hands you a template to fill in. Name, e-mail address and group are required; the belt, phone number, date of birth, sex, weight, age group and weight group come along if the sheet has them.
Checked before anything happens
The file is read in your browser and every row is checked against the club: does the group exist, is that a belt the group has, is the e-mail address an address. You see what would happen — how many to import, how many are members already, which rows have a problem and why, which columns were ignored — and only then press Import.
Nothing doubled
A member who is already in the club is skipped rather than created twice, so the same sheet can be imported again after it has been corrected.