Hello @LeonoraClarke and welcome to the Instructure Community! You mention multiple institutions. Does that mean different Canvas instances? If so, then the trouble might come from some students' inability to access the instance that hosts your course. For example, the instance my college uses does not allow anyone to access it unless they already have a student or employee user account. If your students are able to access the host instance (or if you are using the Free for Teachers instance), then adding a join code will not affect the students who are already accepted their invitations.