Your personal Xchart login represents YOU as an individual and not the organizations that you are a part of.
This is really convenient because you can sign in once, using the same login information, and access multiple organizations that use Xchart.
This happens a lot when in a scenario like the following. Someone is doing their own practice, sometimes contracting with someone else, while maybe also teaching at a school.
If all three of those entities are using Xchart, you can actually join it all.
As a a diagram, it might look something like this:
When you first sign in, you'll be greeted with a screen like this:
You can also quickly switch from one to the other by using the "cog" menu in the upper right and selecting "Switch organizations":
A quick note about your email:
You may have been invited using a specific email address. But it doesn't matter. Your login info is yours. You can change your email address at any time by going to "Profile"
When you change your email on your profile. It changes the email you'll use to sign in and the email address associated with you everywhere within Xchart.