Plagiarism in group assignment

Short answer yes.
Long answer depends on a few factors, such as the guilty party admitting what s/he did; other group members genuinely not being aware of (ie. complicit in) plagiarism, etc.
My advice, look after yourself if someone has landed you in it... I have heard of some nasty outcome from disciplinary committee, so best to avoid any of that hassle if possible.

when handing up a group assignment, do you not sign a declaration saying the work you are submitting is your own and not of others and that you understand the consequences of plagiarism?

If you are signing something you should be happy with its content (all of it including the work of others), i.e. you have read it through and checked that it's ok, including referencing/bibliography.

So in essence although it is severely harsh if you have acted innocently, the onus is kind of on you to make sure you are happy with what is being handed up as a group.

In final year we got accused of plagiarism (wrongly, problem with poor referencing supposedly), but we got to re do it as a group and were deducted marks for it being handed up late, rather than being given a basic 40% pass or even a fail.

monkeymanmatt you don't sound like you're a female with huge tits...

If you know he's plagiarising and you're still handing it up you're gonna need some luck