i dont know is it a bug or my mistake but when i share a media on a group not my own profile it doesnt give any error and seems everything fine. but when i click the image, lightbox pops with 404 page and cant find image.
Its working fine on our demo site, just uploaded a picture under group activity here http://demo.rtcamp.com/rtmedia/groups/test-2066915503/ and after clicking that image it open in lightbox. Kindly try this functionality with WordPress default theme and if its working for you, then you may need to use rtMedia templates in your current theme. Refer more about templates here https://rtcamp.com/rtmedia/docs/developer/templating-system/theme-media-tab/
i feel really stupid. i'm writing just in case some people may miss this like me.. Go wp-admin > rt-media > settings > buddypress > Enable media in group set it ON :D :D