Are non-Jews allowed to sit and participate in a Passover Seder?
If the first day of Pesach falls on a weekday, one may not invite a non-Jewish guest, because of a concern that one might perform a melachah for his sake. If not inviting the non-Jew would cause significant ill-feeling, or the possible loss of a job, it would be permitted.
This is not a halachah that relates to Seder night, but rather a halachah of Yom Tov in general. Therefore if Seder night falls on Shabbat, it would be permitted to invite a non-Jew. Furthermore, if there is no actual ‘invitation’ to one’s home, but rather the participation of a non-Jew at a general seder (such as a communal seder event), it would be permitted.
However, on a night when the special nature of the nation of Israel is singled out, one would have to think twice to ensure that it is appropriate, which depends on individual circumstances.