The consequences of a new member in the opposing team, who was added by the sender of that message, can be divided by this new person's seniority level in the organization you're facing:
If that person is a senior team member, especially a C-level executive, then this is probably a good sign that the deal is going well so far. If the deal is in its early stages, this may convey interest from the buyer. If the deal is in its later stages, this may hint that it has entered the home stretch and is moving towards a resolution.
What if the new person is a mid-level executive? Though less clear-cut, it might also be a good indicator of interest when the deal is in its early stages. At a later stage, it may be that this person was brought in to validate or check a specific domain towards possible resolution.
If a junior employee was added to the conversation, it could go both ways. They might be assigned to a specific task, but it might also mean that your deal has been demoted.