The best way to welcome new people into a church is simply to start off friendly, learn their names, make eye contact, and tell them about the church itself. Make them quickly familiar with key people within the church, and introduce them to the pastor or priest if possible.
When people visit a church for the first time, they often feel nervous, unsure what to expect, and they probably know very little of what the church is all about. To welcome someone into the church, it's best to address these issues in a kind and gentle manner. If visitors are somewhat critical of the church, do not take it to heart. Use any opportunity you have to show off the goodwill of the church, perhaps give them a tour or offer to have lunch together. Be sure to get their names or have them fill out a contact card on one of the church bulletins, as follow-up calls are powerful ways to ensure a revisit from the family or individual. If the conversation calls for it, always feel free to present some of the church's core teachings. Stray away from controversial theological ideas unless the visitor seems comfortable.