If the person you want to add is using Android, you’ll have to create a new group text with SMS.
You can also enter their Apple ID if you don’t know the other information.
That person, as long as they have an iPhone or iPad, will have access to your group text message. If you have a lot of groups, it might be easier to give them nicknames so you can quickly identify them.
You can enable Chat in Gmail by tapping ☰ > Settings then tapping the account you want to chat with (if you have more than one Google account), and finally tapping the box next to “Chat. " If you’ve already created a group text in your regular Messages app, it’s not possible to add someone to the group later—you’ll need to just create another group message if you want to keep your group chat in the Messages app. An alternative is to use an internet-based messaging service like Google Chats or WhatsApp, which lets you add people to group chats at any time.