I wrote a script that uses SMS perfectly and auth token works fine on my personal number. However, I can't sms out of a group number that I am part of. What do I need to check?

What do you mean you are part of a group? A call queue? If so, currently no one in a call queue can send receive SMS via the call queue direct phone number. It will be support in the near future though.

All in all, to be able to send/receive SMS, a user must own the SMS feature of a phone number. See this code for how to detect the SmsSender feature.

/*
  Read phone number(s) that belongs to the authenticated user and detect if a phone number
  has the SMS capability
*/
async function read_extension_phone_number_detect_sms_feature(){
    try {
        let endpoint = "/restapi/v1.0/account/~/extension/~/phone-number"
        var resp = await platform.get(endpoint)
        var jsonObj = await resp.json()
        for (var record of jsonObj.records){
            for (var feature of record.features){
                if (feature == "SmsSender"){
                    // If a user has multiple phone numbers, check and
            // decide which number to be used for sending
            // the SMS message. For simplicity, we pick the
            // first one we find.
                    return send_sms(record.phoneNumber)
                }
            }
        }
        if (jsonObj.records.length == 0)
          console.log("This user does not own a phone number!")
        else
          console.log("None of this user's phone number(s) has the SMS capability!")
    } catch(e) {
        console.log(e.message)
        process.exit(1)
    }
}

https://developers.ringcentral.com/guide/messaging/quick-start