Try 
inside the src/index.js inside bootstrap
bootstrap({ strapi }) {
strapi.db.lifecycles.subscribe({
models: ["plugin::users-permissions.user"],
// Send email to new user
async afterCreate({ params }) {
const {
data: { email, username },
} = params;
try{
await strapi.plugins['email'].services.email.send({
to: `${email}`,
from: `${process.env.SENDGRID_DEFAULT_FROM}`,
replyTo: `${process.env.SENDGRID_DEFAULT_TO}`,
cc: 'valid email address',
bcc: 'valid email address',
replyTo: 'valid email address',
subject: `${username}, Welcome to the ....`,
html: `<p>${username}</p><p>Welcome to ....</p>
<p>${process.env.CLIENT_BASE_URL}/confirm-email</p>`,
})
} catch(err) {
console.log(err);
}
},
});
}