Haiku nightly doesn’t appear to include the usermod command line utility, nor a secondary -G flag to useradd, for reliably adding users to groups.
I think I will work around this gap by carefully tweaking /etc/group directly, though this feels fragile.
Update
I wrote a GNU awk script to assist with carefully, automatedly, adding users to groups in Haiku:
Keep this good job up, mate.
Maybe you could start contribute to Haiku, the posix-userland-backend-idk-thoose-cli-programs-you-know needs plenty love to make it realible. No everybody needs them tho, but if somebody needs, it should work.
Hope you will get your trac email soon, as the forum not the best place to report these problems.
Maybe an admin can check the trac and activate your account manually. You can ask them on IRC…
You are aware that haiku ist atm single user operating system, also multiuser is prepared in some areas i am not shure how much is usable at the moment.
We won’t know if no one tries to use it, however 
You never know, we could be talking to the guy who gets m/u implemented.
There is quite a bit of userland stuff that can be worked on and played with in the mean time.
You surely can. Let’s hope that they can respond in time, before one eventually loses interest and leaves for good.