Once you join the group, there will be a submit to Gallery button at the top of the page. Otherwise, you can go to the deviation you want to submit, and there will be a menu on the right side of the screen where you can choose to submit it to a group. If the group name doesn't immediately come up, you can type the name into a search bar when you click the round button to "Type a Group Name". *^_^*
For casual critique, you can submit it to the Critique folder. If you're looking more accurately for critique from a particular person (like one of the admins~) you're welcome to tag us in the deviation description.