They are part of the WPSL. As you mention, an amateur league, primarily during the Summer, but they start exhibition and games actually in the late Spring. Most teams carry pretty large rosters, and so are flexible when their players arrive, to a certain extent. About 120 teams in the league.
Albion Hurricane out of Houston just added a team, San Antonio just added the Blossoms, and Lonestar SC and FC Dallas are all in the "South TX" Division.
WB '99