I have compiled a list of US LLM groups on lawyrs.net including signup links for those who haven't joined the lawyrs network yet. I have also listed some non-LLM groups, which I thought could be useful for other LLM-Guide users. If you find other interesting groups, please post them below.
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LLM Groups:

LLM-Guide Group (66 members)
www.lawyrs.net/groups/6/llm-guide
Signup link: www.lawyrs.net/llmguide/6

NYU LLM 2009 (63 members)
www.lawyrs.net/groups/33/nyu-ll.m.-2009
Signup link: www.lawyrs.net/llmguide/33

Columbia LLM 2009 (32 members)
www.lawyrs.net/groups/30/columbia-llm-class-09
Signup link: www.lawyrs.net/llmguide/30

Harvard LLM 2009 (25 members)
www.lawyrs.net/groups/29/harvard-llm-class-of-2009…
Signup link: www.lawyrs.net/llmguide/29

Duke LLM 2009 (24 members)
www.lawyrs.net/groups/31/duke-2009-llm
Signup link: www.lawyrs.net/llmguide/31

Georgetown LLM Class of 2009 (47 members)
www.lawyrs.net/groups/25/georgetown-llm-class-of-2…
Signup link: www.lawyrs.net/llmguide/25

Northwestern LLM 2009 (21 members)
www.lawyrs.net/groups/55/northwestern-university-l…
Signup link: www.lawyrs.net/llmguide/55

Fordham LLM 2009 (17 members)
www.lawyrs.net/groups/38/fordham-llm-20082009
Signup link: www.lawyrs.net/llmguide/38

UCLA LLM 2009 (14 members)
www.lawyrs.net/groups/27/ucla-llm-class-of-2009
Signup link: www.lawyrs.net/llmguide/27

Boalt Hall LLM 2009 (10 members)
www.lawyrs.net/groups/59/boalt-hall-llm-class-of-2…
Signup link: www.lawyrs.net/llmguide/59

Boston University LLM 2009 (9 members)
www.lawyrs.net/groups/40/boston-university-llm-200…
Signup link: www.lawyrs.net/llmguide/40

USC LLM 2009 (7 members)
www.lawyrs.net/groups/39/usc-llm-20082009
Signup link: www.lawyrs.net/llmguide/39

UPenn (5 members)
www.lawyrs.net/groups/67/upenn-llm-class-09
Signup link: www.lawyrs.net/llmguide/67

Cornell (2 members)
www.lawyrs.net/groups/47/cornell
Signup link: www.lawyrs.net/llmguide/47

Other groups:

Law Firm Networking
www.lawyrs.net/groups/66/law-firm-networking
Signup link: www.lawyrs.net/llmguide/66

In-House Lawyers
www.lawyrs.net/groups/54/in-house
Signup link: www.lawyrs.net/llmguide/54

Tax Consultants
www.lawyrs.net/groups/19/tax-consultants
Signup link: www.lawyrs.net/llmguide/19

Criminal Lawyers
www.lawyrs.net/groups/15/criminal-lawyers
Signup link: www.lawyrs.net/llmguide/15

Intellectual Property Law Group
www.lawyrs.net/groups/21/intellectual-property
Signup link: www.lawyrs.net/llmguide/21

Human Rights
www.lawyrs.net/groups/14/human-rights
Signup link: www.lawyrs.net/llmguide/14

WTO Law
www.lawyrs.net/groups/49/wto-law
Signup link: www.lawyrs.net/llmguide/49

WTO and Environment
www.lawyrs.net/groups/50/wto-and-environment
Signup link: www.lawyrs.net/llmguide/50

Lawyers’community in China
www.lawyrs.net/groups/51/lawyers-community-in-chin…
Signup link: www.lawyrs.net/llmguide/51

American Lawyers
www.lawyrs.net/groups/17/american-lawyers
Signup link: www.lawyrs.net/llmguide/17

French Lawyers
www.lawyrs.net/groups/22/french-lawyers
Signup link: www.lawyrs.net/llmguide/22

Italian Lawyers
www.lawyrs.net/groups/8/italian-lawyers
Signup link: www.lawyrs.net/llmguide/8

English Lawyers
www.lawyrs.net/groups/16/english-lawyers
Signup link: www.lawyrs.net/llmguide/16

Pakistani Lawyers
www.lawyrs.net/groups/45/pakistani-lawyers
Signup link: www.lawyrs.net/llmguide/45

Africa Business Law
www.lawyrs.net/groups/18/africa-business-law
Signup link: www.lawyrs.net/llmguide/18

Brazil
www.lawyrs.net/groups/65/brazil
Signup link: www.lawyrs.net/llmguide/65

Abogados Mexicanos
www.lawyrs.net/groups/46/abogados-mexicanos
Signup link: www.lawyrs.net/llmguide/46

Oxford Alumni
www.lawyrs.net/groups/23/oxford-alumni
Signup link: www.lawyrs.net/llmguide/23

Delhi University
www.lawyrs.net/groups/72/
Signup link: www.lawyrs.net/llmguide/72

I can’t believe it’s a law firm – Fans of Lionel Hutz
www.lawyrs.net/groups/48/i-cant-believe-its-a-law-…
Signup link: www.lawyrs.net/llmguide/48

George Washington 2008/2009: http://www.lawyrs.net/llmguide/75/

Hello, there is also a group for Indian lawyers: http://www.lawyrs.net/llmguide/76 ...hope you join!