Well if they aren't prepared to compromise then you really have no option but to bring the matter to your course leader as I would imagine if they turn their work in and you don't, it will be you who is penalised.
A group is meant to work together for the benefit of the whole team, not for a majority and therefore it really doesn't matter what the other 4 want when it only serves to benefit them.
You're going to have to be more assertive; I'd send a group message to them all along the lines of....
"Hello everybody,
I understand that we have conflicting schedules and what suits one doesn't suit the rest but unfortunately this project is due very soon and we have no more time to waste.
On the basis that we are a team for the purpose of this assignment I propose that we reach a compromise so as to allow everyone to participate without exclusion or exception.
I gather from our previous conversations that weekends are very precious to you and just like you I'd rather not be in the position of having to do collective group course work on a weekend, but the reality is it has to be done.
I will make myself available insert date and time because I care about this course, I am serious about my achievements and I want to do well, hence why I enrolled on the course in the first place.
If for whatever reason you feel unable to commit to insert date let me know asap so I can contact our course leader to inform him/her of the issue - I don't want to lose marks to accommodate your other commitments, and I am sure you would feel the same way."
There you go, polite but to the point; Don't be a bloody push over, speak up. They won't give a damn if you fail, stop thinking of their needs - You offered a compromise, they refused, contact your course leader!