LGBT means lesbian, gay, bisexual and transgendered. When you speak of the parenting and family community you also need to consider those considering parenthood, those formerly married, straight spouses, non-biological parents, foster parents, step families and blended families. Think about whom you want to include and why. Think of ideas for press and publicity. Often word of mouth is very powerful.
Personal invitations are powerful.
Mailing flyers and publishing notices are helpful too. Be realistic on what you can and cannot do. A good indicator of time required to start up a group is 40 to 50 hours spread out over 1 to 2 months. Think about what you can do and make a short list. Have fun and come up with something catchy that reflects what you want the group to be and what your environment is all about.