1 How do I get the NightlyBuilds group and thus access the NightlyBuilds forum?
First click on your profile picture in the top right corner and then go to the bottom of the list and click on User Groups.
After that you will see an overview of all the usergroups, find the Nightly Builds group and click on Join Group.
After that you should find the forum Nightly Builds in the forum overview.
2 How do I install the NightlyBuilds version?
First, go to your Steam Library and right-click on LOTUS Simulator, then go to Properties.
There you go to the top of the BETAS tab and select NightlyBuilds from the list (Nightly Builds, fixing issues or creating new ones. Get always the most recent release candidate of LOTUS.)
Note: It is ALWAYS important to read the patchnotes of the NB version!