Amanda, this is Emily.
Are you also an admin user besides being a channel owner? If not, please have your organization's admin user check and make sure your organization's settings is set to allow external user/guest users.
Also, on the guest users' side, make sure they are using the Teams for Work or School edition, not the Teams for Home app. They can download it from here: https://www.microsoft.com/microsoft-teams/download-app
what if they only have a personal Microsoft account? Will the teams for work or school edition work for them? My company is new to teams and we have a lot of freelancers and contractors we need to add to channels, teams and be able to have 1 on 1 chats with. but it seems everyone I try to add (I am the admin by the way, and our org settings are set correctly), there's issues. either they get login errors because they are not a part of our tenant, or they can see teams, channels and chats on their mobile but not on desktop. or they can't see anything but chats? in both the web version and desktop.