Just think of it this way - the courses here are public courses; anyone can play at any time a slot during a tee time is available. The tee times are not owned by anyone in that group so if a slot is available, go for it.
I joined a threesome one day and one of them got kind of ****y about it; guess he had some kind of "bromance" going on with the other two. I looked at the scorecard and asked him to show me where it showed him to be the owner of the course. Needless to say we didn't bond during that round, nor did we ride together.