Npc Greeter? Why not? I have made a single prim npc greeter. It greets the person and sends them a group invite to your group. Just remember to change the group on the object that you want them to be invited to. Also edit the script and change the wording to say what you want it to say. You can put any npc card in the yellow circle prim and that npc will show up (as long as it is the only one in there) If you need any help at all just ask me! I almost forgot. Here is the script im working on to stop greifers: -
https://opensimworld.com/library?view=97 It is very advanced - like a swiss watch - so be careful and know what you are doing.