If you are unable to invite for teachers to a class then that means you have hit a limitation. You can only have 50 teachers per class. If you invite more than 50 then they can't join the the class.
There is also a limit to how many teacher invitations you can extend in 24 hours, which is 500. Be careful not to approach these limits or be labeled as spam. Wait until the next day to add teachers so you don't get an error.