Not all History teachers are members of the HTA.
At my school 2 teachers are but we have 7 teachers who teach history - although the other 5 aren't predominately history teachers being either English/History or HSIE trained.
The brochures you mention, however, are sent to every school regardless of whether there are individual or institutional members at the school.
For HTA conferences - one was last Friday on the Extension Course - while there is one in August on 'niche' topics - there are different charge rates depending on the membership of the person. Individual or institutional members pay less for the days than those people who are not members.
The study days are open to students regardless of the membership status of their teacher or school.
The ones at Milsons Point are well worth attending as the people are very knowledgeable.
I have never taken students to the ones run by the HTA personally but I have made the students aware of them. The kids that have gone have spoken highly about them.