Try posting a notice at a local yarn store and advertising on social media to help find members for the group. Think about whether you want to include beginners, experienced knitters or both. Once enough people have registered their interest, select a date, time and location for the first meeting. Have each member introduce herself and describe her knitting experience, as well as what she hopes to get out of the group.
An informal discussion at the first meeting can help to identify ways to organize the group, for example, if a member suggests a venue or volunteers their house. Libraries and other institutions may have rooms that are available to rent. Remember that the location needs to have enough seats for a varying amount of people from week to week. A quiet cafe can work well as a venue, provided the manager does not mind a group of people staying in one place for a long time. Try to pick a location that is easy to reach on public transport and has enough parking.
Ask other people to help with matters such as finding venues, organizing food and communicating with other group members. Rotating responsibilities every few months gives every member the chance to contribute equally. Try to ensure that the group meets at the same time and place every week so that members have the best opportunity to work the group into their routines.