Gonna try this again because my last answer wasn’t robust enough. ㅠㅠ
The message “항상 우리 곁에 있어줘서 고마워요” roughly translates to “Thank you for being with us all the time.”
항상 = Always
우리 곁에 = beside/by us
있어줘서 = you are ('줘서' is to give, '있어' is to be; you are 'giving' your
고마워 = thank you
But they’re not referring to literally being beside them (obviously). They’re thanking you as a fan for giving them your support in buying their album and listening to their music. In English, we similarly use the term “Thanks for standing by me”!