Group Inviter
Second Life group inviter is a scripted agent (bot) which sends direct group invitation to other Second Life residents.
This is useful to automatically invite visitors to specific group, or invite all buyers of the certain good or service to support group.
You may host your own bot to sit on your land by buying software and installing it on your PC, or by renting SL bot. Renting is usually cheaper considering that you don't need to keep your PC around the clock.
Due to LSL limitations it's impossible to send direct group invitations using in-world objects. Only way is to use the scripted agent (read more here).
Existing bot software
Refer to Group inviters comparison to compare existing bots and their possibilities.