These are good people and are planning to marry in the Church. So in the mean time, should they NOT receive Holy Communion and go to Confession?
Is it a grave sin for a civilly married couple (never remarried to another) to receive the Holy Eucharist?
Assuming that they are Catholics, until they are married in the Church their current marriage is invalid. So if they have marital relations, they are committing mortal sin. If they are living in a celibate way, they may go to Confession and receive the Eucharist, but preferably in a Church where they are not known to have been married only civilly, to avoid scandalizing anyone.
Fr. Vincent Serpa, O.P.