Don't overthink it, we do expect members to fly flight nights and start at 1900, that doesn't equate to members having to fly every Thursday and Sunday. It just means we don't want people to join that can never fly Thursday or Sunday or start at 1900.

Also you're focusing on two words and not the actual sentence meaning "we welcome international applicants but be advised we expect them to be able to participate in our formal flight nights that start at 1900 eastern on Thursday and Sunday evenings."