To open up your group so that members can submit,
Go to Admit Area at the top of your group page.
Go to "Manage Members" on the upper left of that page.
Click on "Members" on the left
Under "On Gallery Members"
make sure the drop down box says either
"Are subject to vote"
or
"Are automatically Approved"
You can choose to do this for every individual folder you created and customize it however you like. It took me awhile to get this too but that should open it up to members who joined your group and can now submit work to you. The subject to vote part means that you have to approve each image that people throw into this group. I have mine for now being accepted automatically cause my group is small. You can also adjust how many photos you want submitted into each folder. GOod luck and I hope it works for you.