To create users on startup with a blank database:

import org.codehaus.groovy.grails.web.servlet.GrailsApplicationAttributes

class BootStrap {

def init = { servletContext ->

new Role(description:"Administrators", authority:"ROLE_ADMIN").save();
new Role(description:"User", authority:"ROLE_USER").save();

def ctx = servletContext.getAttribute(GrailsApplicationAttributes.APPLICATION_CONTEXT);

def authService =  ctx.getBean("authenticateService");
def pass = authService.passwordEncoder('aPasswordGoesHere');

def config = authService.securityConfig;
def defaultRole = config.security.defaultRole;

def role = Role.findByAuthority(defaultRole);

def adminRole = Role.findByAuthority("ROLE_ADMIN");

def p = new User(username:"admin"
,passwd:pass
,enabled:true
,email:'test@example.com',
,userRealName:"Default Admin");

role.addToPeople(p);
adminRole.addToPeople(p);

if (p.save())
println("Created default admin");
else {
p.errors.allErrors.each {
println it
}
}
}