Hi Tracy, I'll be happy to answer these for you.
Right now, there is currently not a way to group message your team or company admins, unless they have also done something like purchased a ticket or filled out a volunteer application. If they are in your system only as a team or company admin, you would have to message them individually.
As far messaging a select group of applicants, yes you can do that. For this situation, you can create a tag such as "cert of liability needed". You could then apply that tag to the ten approved applicants who have not submitted their certificate of liability yet. Once you have that tag applied, you can go into the group message feature in the Artists, Vendors, and Exhibitors section, and select that tag in the "tags to include" filter. Then, only the groups who have that tag applied would receive that message.