Hi,
I understand the external user still cannot join after you checked the invitation.
As the forum’s AI recommended, please start with the exact error message. Could you share what appears when they try to join, whether they are using the Teams app or a browser, and whether they are signed in with a work/school account? Please also confirm whether other external attendees can join the same meeting.
In the meantime, check the situation that applies:
- If they are waiting in the lobby, the organizer should enter the meeting and admit them. If the message says the meeting is locked, the organizer needs to unlock it before they retry. On a phone or tablet, use the Teams mobile app because joining through mobile web browsers is not supported.
- If they see “Sign in to join this meeting,” ask them to sign in with their Teams work/school account. If joining without an account is required, your IT admin should review the organization’s anonymous join setting and the meeting policy assigned to the organizer. In Teams admin center, these are under Meetings > Meeting settings and Meetings > Meeting policies > the organizer’s assigned policy > Meeting join & lobby. See Manage anonymous meeting access. Policy changes can take up to 24 hours.
- If they see “Sign in with a different account to join this meeting,” check that they are using the invited account. This message can also occur when external access is restricted and anonymous join is disabled. Both organizations’ admins should check the relevant domain permissions and user-level external access policies. See Errors when external participants try to join a Teams meeting.
If those checks do not explain the issue, the attendee’s IT admin should also review their assigned meeting policy’s “People can join external meetings hosted by” setting. Microsoft documents this separate restriction in Manage access to external meetings.
These checks do not require enabling every external access option. The appropriate change depends on how the attendee is joining and which restriction applies. Let me know how it goes after trying these steps.