@Icsab
Here is information on how to join a Microsoft Teams meeting: https://support.office.com/en-us/article/join-a-meeting-in-teams-1613bb53-f3fa-431e-85a9-d6a91e3468c...
It sounds like you might be scheduling a channel meeting. If that is the case, your guests may not join a channel meeting unless they are guests of the team. So you need to invite them to the team and then they will have the ability to join a channel meeting. A channel meeting is a meeting that is taking place in the team whereas a regular Teams meeting is a scheduled meeting that anyone you choose can attend.
If you schedule a meeting in the Microsoft Teams and do NOT choose a channel to host it in, it will be a Teams meeting and anyone you give that link to can attend. If you schedule it in Outlook, anyone you invite via that Outlook invite can attend the meeting.
If you create a channel meeting (this happens when you 'add a channel' at scheduling), only the people in the channel (the team) will be able to attend. Anyone not a guest or member of the team will not be able to access the meeting.
I hope that helps.
