Since we're still a new community, we spend a lot of time trying to find and integrate new people into our core of regulars at the house. Newcomers are still a fairly frequent event at our house, given the breadth of events we host, but a lot of them are one-off attendands who come for a specific event rather than to join the community. We still do not have a formal protocol for welcoming new comers (and we're not sure that having one is a great idea), so we apply the standard rules of Russian hospitality to the situation.
Our first step is to welcome the newcomer personally, introduce ourselves as the hosts and welcome him to the house. Showing the guest around the house is a tradition that we follow strictly to help the new person feel more at home. We then let them integrate into the event we're hosting organically. If the person knows none of the guests, we may introduce him or her to some new people, but in a lot of cases, we let people make their own introductions, if they look like they're comfortable doing so. In the course of the event, we'll check up on the person, but will not single them out for too much attention, as that might make people uncomfortable. At some point, we try to strike a conversation to get to know the person. The goal here is two-fold. We want to start a relationship that hopefully lasts beyond one event, make the person feel like they have friends in the house and make them want to come back. Secondly, we want to learn how they found out about our house, what specifically brought them to this event, and what they'd like to see in the future in terms of events.
When the guest is ready to leave, we make sure we personally bid goodbye to them and see them off. We then try to follow up with them, make sure they enjoyed their time, and personally invite them to future events. Sometimes people come to one event and we never see them again, but most of the time, people come back.