Any person can participate in competition, with no limitation (including speedcubers from other countries), if he/she accepts WCA Regulations.
Preregistration available on WCA website.
If you already have participated in an official competition, you can use the search function which will fill the information stored in the database. You can then fill the rest.
Please note that the purpose of the preregistration is not only to reserve you a spot in the competition, but also very importantly to give the organisers a good estimation of the number of people they have to expect. Please don’t wait until the very last minute to preregister, otherwise the organisers might not be able to offer enough room, food, etc.