If you don't plan on providing any financial aid for participants you should open an application process at least a month (ideally two) before the event. Send people acceptance/rejection e-mails at least a week before the event and make a waiting list: you may need to adjust your groups if people can't make it. Always make sure attendees confirm that they will attend.
If you organize an event and expect to have people from many different countries, you should start as early as possible. There is a chance that you will have people who need visas. In this case, you need to let people know if they are invited at least one month before the event. Don't forget to make a waiting list: you may need to adjust your groups if people can't make it. Always make sure attendees confirm that they will attend.
Visa invitation letter
It is also possible that you will need to provide an invitation letter for the participants who require visas. It might look like this:
Through this paper we certify that [name of participant], born [birth date of participant], living at [country], [address], with passport number [passport number of participant], valid from [date] to [date], is invited during the period of [date] to [date] to participate in the Django Girls workshop organised by [name of some legal entity or you] with the address: [address]; phone: [phone], e-mail: [email].
[here you can put some info about who organises the event - if it a part of conference you should put some info about it here]
Free food and social activities will be provided by organisers during the period of stay.
We kindly request that you grant a free visa for this particular participant of the Django Girls workshop.
You will need to send this letter (signed!) to the participant. You should also scan the signed papers and send the copy via e-mail.