Hi @Kellee_Kosiorek1 - Welcome to the Asana community! I am fairly new here as well, but am familiar with your issue. I just set up some guest users in our organization! Guest users must have a separate email domain from your organization. A regular Gmail account should not prevent you setting a user as a guest.
To start, you can review the permissions for all of your members and guests through the Admin Console for your organization. On the left of the Admin page, choose Members. This will bring up a list of all users currently linked to your organization. Be sure to select All in the dropdown, or you may not see everyone.
Here you can select each member and then set their individual access. All non-members should be marked Guest in this area.
There is additional information regarding user permissions in this thread.
Cheers!
Brandi