The Maui group of the Sierra Club of Hawaii has an outing schedule here:
It is updated every three months.
You do not need to be a member to come on an outing, but you generally have to register with the leader of the hike in advance. Most hikes have a recommended donation of $3 for members and $5 for non-members.
There is also a Facebook page to make it easier to find out about upcoming hikes:
Join the Facebook group to be notified of new hikes and to include the hike schedule in your own calendar (e.g. Google Calendar).
Another group that puts together a schedule of hikes for several months in advance is Mauna Ala. Here are a couple of recent schedules:
- Mauna Ala hiking schedule – August 2015 – February 2016
- Mauna Ala hiking schedule – February – September 2016
- Mauna Ala hiking schedule – October 2016 – April 2017
- Mauna Ala hiking schedule – April – October 2017
- Mauna Ala hiking schedule – October 2017 – April 2018
- Mauna Ala hiking schedule – May 2018 – October 2018
- Mauna Ala hiking schedule – October 2018 – April 2019
- Mauna Ala hiking schedule – May 2019 – October 2019
- Mauna Ala hiking schedule – November 2019 – April 2020
Note that the latest one may not be here. Unlike Sierra Club, they don’t require advance registration for a hike and they don’t ask for a donation for the hike, but they require that you pay a $5 annual membership fee (or come as a guest of a member).
Friends of Haleakala National Park
They organize monthly service outings in the crater including two nights at cabins:
Auwahi Forest Restoration Project
The project includes a monthly service outing to plant natives plants in the Kahikinui area above the wind turbines: