Working Groups are typically created with a charter that describes the specific problem or deliverables (a guideline, standards specification, etc.) it has been formed to address. WG charters state the scope of work for group, and lay out goals and milestones that show how this work will be completed.
There are typically over 100 active Working Groups at any particular time, and each is part of an Area that is most relevant to its work. While Working Groups are generally expected to be short-lived in nature, some provide ongoing improvements of established protocols. Upon completion of its goals and achievement of its objectives, a Working Group is concluded. The IETF Datatracker provides a complete list of all active working groups.
If you are interested in contributing to a particular Working Group, the information about participating in the IETF may be helpful.