These days we can’t bring the Pesach sacrifice, because there is no Temple, but we still hold a meal in order to hold a celebration. But does this mean that because the meal doesn’t include the paschal sacrifice, a ger (convert) could now join in a uncircumcised state? And what if the Temple didn’t fell and we would still bring the Pesach sacrifice and eat it at our meal, would these people be still invited to join?
Firstly, a male cannot convert without circumcision, so he is simply a gentile. Their prohibition is only regarding the paschal sacrifice, so today, when we don’t have the sacrifice, they can participate in the Seder with Jews. When we did and will have the sacrifices, the uncircumcised gentile can attend the seder but just not eat from the sacrifice.