To RSVP by text, send one message to the number on the invitation that says who you are, names everyone replying, gives a clear yes or no and adds anything the couple asked for, such as dietary needs.
Text only when the couple has invited it: the invitation asks for a text, gives a keyword for a text-reply service (include that word) or arrived by text itself. Where it gives only a phone number, the advice differs: Reader's Digest treats a number given for replies as an invitation to text, while the Emily Post Institute says to call and speak to someone. If in doubt, call. A reply card should still go back in the post. Open with your name, as the Emily Post Institute advises for any text to someone who may not have your number.
For example: "Hi Anna and James, it's Priya Shah. Arjun and I would love to come on 14 May. Two of us, both vegetarian. Thank you for inviting us." To say no by text, use the short note under how to decline.