I guess it's a good way to get my work seen... I got like 29 favs within two days of uploading to a group. Watchlist-wise it's a cool way to see deviations beyond the styles that I usually watch, but on some days the sheer numbers amount to spam.
I like submitting to groups, but some groups just have too many submissions, and I rarely get to go online, so once I log in I see 670 deviations, like now. But now they have this stack deviation thing, so it makes my sorting through deviations life easier.
Yes, if you want to 'contribute art' to a group, you have to join it. Unless the group 'asks for your permission for your piece to be displayed in the group', then your work will show in the gallery without you needing to join it.
I like groups and I am in several. You do get a lot of deviations, but you also get a lot of publicity. Plus there are groups with less devs and some with more, so one can join the ones with less members and devs. I think some people just join too many groups at once and then get flodded.
there are some groups that do not spam my box even they have over 1000 members. although they accept most submissions, they have selection process after certain period, and only the good ones will be sent to the members. so, they are not always helping our promotion, but when they select your work, you will get over 100 faves sometimes. it works like a contest.
other good one has pre-selection process. in other words, they are very picky about what they put in their gallery or collections. if your style fit to the group, it would be your heaven.... otherwise you may end up hating the group. usually, you can see so many people are complaining about their DECLINED works in the group front page.
i love small groups. we can share the idea about what we like. since it is small, i can check all incoming works and enjoy them. usually these groups are focusing on such narrow topic that only small numbers of people like to talk about it. i am not a member of it, but i just found one group only about POCKY, a japanese sweet snack. i was so amazed by the idea and also broad styles of works in their gallery about pocky.
all other groups i am in are flooding my box with so many deviations everyday. thank you for new stack feature, i can check them out when i have a time. before i had to delete all or unchecked "deviation" to maintain the time for my friends.
personally, i think everything happening in dA is GIVE and TAKE.