If the youth event in the home is church-sponsored, permission slips would be needed—for example, if a Sunday school teacher invited the students in her Sunday school class to her home. If a church worker informally invited a group of young people over for pizza but it was a personal invitation and not under the auspices of the church, then no permission slip would be required. In that case, any legal or liability issues that might arise would be the responsibility of the adult worker/homeowner rather than the church.
Our general guideline is that if an event is held on church premises (thus is similar to a public meeting where all are invited) a permission slip is not required. If a church-sponsored event is held off site, in an environment that is not church controlled, permission slips would be required. The exception would be Boys/Girls Clubs which typically are held on church premises but still require a permission slip for participation.