I've got a presentation tomorrow and my group are absolute *******s, I need a legit excuse to get out of it!? Help?!
I would still attend and do the presentation, try my hardest so at least your marks aren't deducted. If the group has been failing to perform throughout, and you have the evidence, speak to the course supervisor/lecturer right after.
Many might not know this. But if a group fails to perform, and you yourself tried to prevent this, attempted to help them and still tried your hardest. This is grounds for you still being able to achieve a mark as if it was a solo presentation. Same goes for when specific members in a team purposely fail to-do any work. They can be penalised and marks will not be lost for yourself/rest who actually tried. Though do try to communicate this in advance next time.
Unless you're too ill to go on in or a loved one is in hospital/ dead (and they will ask for evidence) there is no good excuse.
I understand you got stuck with crap group, but suck it up. Do you really think you'll be able to just get out of things and not go to your job in the future or do a certain task because your colleagues weren't great at helping you? No. This is part of what group work is all about at university. You will encounter rubbish people in the work place and you need to develop skills to tackle these problems now while in university do you're prepared for the future.
I don't know what your university's policy is on not attending assessments, but at mine if you didn't turn up for a presentation is was an automatic fail and you would only have yourself to blame.
Hopefully you'll learn something from this experience.