It can happen that the invite link you clicked on has already expired or the whole group has been dissolved by today. Please ask for a new one from a member or ask them to add you directly without an invite. Note that invitations are only processed while both ends are online.
If the invite link does work for others, we can proceed with the investigation. Are the two of you on the same chatmail server? If no, have you tested that you can send private messages back and forth with users on that host? Lost or blocked secujoin messages may result in such a case, in which case deleting & rejoining might sometimes help (not sure whether you would need a new invite).