Admin panel:

Only invited user can register in strict mode,
Set mail server in admin panel,
Switch strict mode in admin panel,
Invite people to system in admin panel

models/settings.js

@@ -0,0 +1,111 @@
+Settings = new Mongo.Collection('settings');
+
+Settings.attachSchema(new SimpleSchema({
+  strict: {
+    type: Boolean,
+  },
+  'mailServer.username': {
+    type: String,
+  },
+  'mailServer.password': {
+    type: String,
+  },
+  'mailServer.host': {
+    type: String,
+  },
+  'mailServer.port': {
+    type: String,
+  },
+  'mailServer.from': {
+    type: String,
+    defaultValue: 'Kanban',
+  },
+  createdAt: {
+    type: Date,
+    denyUpdate: true,
+  },
+  modifiedAt: {
+    type: Date,
+  },
+}));
+Settings.helpers({
+  mailUrl () {
+    const mailUrl = `smtp://${this.mailServer.username}:${this.mailServer.password}@${this.mailServer.host}:${this.mailServer.port}/`;
+    return mailUrl;
+  },
+});
+Settings.allow({
+  update(userId) {
+    const user = Users.findOne(userId);
+    return user && user.isAdmin;
+  },
+});
+
+Settings.before.update((userId, doc, fieldNames, modifier) => {
+  modifier.$set = modifier.$set || {};
+  modifier.$set.modifiedAt = new Date();
+});
+
+if (Meteor.isServer) {
+  Meteor.startup(() => {
+    const setting = Settings.findOne({});
+    if(!setting){
+      const now = new Date();
+      const defaultSetting = {strict: false, mailServer: {
+        username: '', password:'', host: '', port:'', from: '',
+      }, createdAt: now, modifiedAt: now};
+      Settings.insert(defaultSetting);
+    }
+    const newSetting = Settings.findOne();
+    process.env.MAIL_URL = newSetting.mailUrl();
+    Accounts.emailTemplates.from = newSetting.mailServer.from;
+  });
+
+  function getRandomNum (min, max) {
+    const range = max - min;
+    const rand = Math.random();
+    return (min + Math.round(rand * range));
+  }
+
+  function sendInvitationEmail (_id){
+    const icode = InvitationCodes.findOne(_id);
+    const author = Users.findOne(Meteor.userId());
+    try {
+      const params = {
+        email: icode.email,
+        inviter: Users.findOne(icode.authorId).username,
+        user: icode.email.split('@')[0],
+        icode: icode.code,
+        url: FlowRouter.url('sign-up'),
+      };
+      const lang = author.getLanguage();
+      Email.send({
+        to: icode.email,
+        from: Accounts.emailTemplates.from,
+        subject: TAPi18n.__('email-invite-register-subject', params, lang),
+        text: TAPi18n.__('email-invite-register-text', params, lang),
+      });
+    } catch (e) {
+      throw new Meteor.Error('email-fail', e.message);
+    }
+  }
+
+  Meteor.methods({
+    sendInvitation(emails, boards) {
+      check(emails, [String]);
+      check(boards, [String]);
+      const user = Users.findOne(Meteor.userId());
+      if(!user.isAdmin){
+        throw new Meteor.Error('not-allowed');
+      }
+      emails.forEach((email) => {
+        if (email && SimpleSchema.RegEx.Email.test(email)) {
+          const code = getRandomNum(100000, 999999);
+          InvitationCodes.insert({code, email, boardsToBeInvited: boards, createdAt: new Date(), authorId: Meteor.userId()}, function(err, _id){
+            if(!err && _id) sendInvitationEmail(_id);
+          });
+        }
+      });
+    },
+  });
+}