There are two separate signup availabilities for a giveaway.
There is the member signup. This is for those that just want to download the free gifts that are available.
Then there is the contributor or JV partner signup. This is for those who have available, and wish to offer, free gifts to others and at the same time build their list.
You cannot sign up to be a member of a giveaway until the giveaway opens to members which will be at the date and time which is set by the person(s) running the giveaway.
Whereas those that wish to be contributors to the giveaway can sign up as soon as the giveaway is put on the internet. This gives the contributor the time needed to upload his gift to the giveaway before the giveaway opens to members.