Since this document is meant as a guide not only to maintaining and growing LUGs but also to founding them, we should, before going further, discuss what LUGs already exist.
There are several LUG lists on the Web. If you are considering founding a LUG, your first task should be to find any nearby existing LUGs. Your best bet may be to join a LUG already established in your area, rather than founding one.
As of 2016, there were LUGs in 43 US states, seven of Canada's ten provinces, all six of Australia's states plus the Australian Capital Territory, in 76 locations in India, and over 100 other countries, including Russia, China, most of Western and Eastern Europe, and many parts of Africa, Asia, South America, Central America, Oceania, and the Caribbean. (This does not include Linux groups internal to Meetup.com.)
While (most) LUG lists on the Web are well-maintained, likely they don't list every LUG. If considering founding a LUG, I suggest, in addition to consulting these lists, posting a message to comp.os.linux.announce, comp.os.linux.misc, or an appropriate regional Usenet hierarchy, inquiring about nearby LUGs. You should also lodge a query (mailing list post, comment during a meeting) at any existing LUG you are aware of anywhere near your area, about LUGs near you. If no such (nearby) LUG exists, your postings will alert potential members to your initiative.
Carefully balance convenience against solidarity: If a LUG exists in your metropolitan area but on the other side of the city, starting a new group may be better for convenience's sake. On the other hand, joining the other group may be better for reasons of unity and solidarity. Greater numbers almost always means greater power, influence, and efficiency. While two groups of 100 members each might be nice, one with 200 has advantages. Of course, if you live in a small town or village, any group is better than none.
The point is that starting a LUG is a significant undertaking, which should be commenced with all relevant facts and some appreciation of the effect on other groups.