I did it before and I have permissions set, but all of a sudden when I try to start a conversation I get "You do not have permission to view this page or perform this action." I'm the admin, I have everything set to "allow."
If you use your groups correctly then the original value from the Registered users group would have applied, unless you use never.
There is no distinction between primary and secondary user groups.
It is recommended however that all users have the Registered user group set as their primary user group and any additional groups they are members of are set as secondary groups. This makes it easy to manage permissions due to the cumulative nature in which they are applied.
I was driving today and slapping my forehead as soon as I realized that it refers to the number of participants, not the number of conversations. Obviously if it's set to 0 you can't invite anyone.
P.S. The lesson for me is if I create a new user group it defaults to 0 (I didn't use the registered user group as I should have to create a secondary group so there was nothing to inherit). You have to be sure to manually set the value to "5" or whatever. Something to watch out for.
We assume that when you're creating additional groups that they'll be used as secondary groups, so all the values are set to their "minimum", so you have to explicitly grant them.
For the most part, the 2 basic groups are for registered users and guests. Additional groups are then used to grant additional things to special members (allow) or potentially remove options for bad members (never).