Sejeong wasn't added to Gugudan lol, she debuted with them since beginning, Yeonjung was added in WJSN and went smooth, if they planned to debut her in Everglow is better that they debuted them first since they managed to get a decent fanbase on their own and when Yena join, they won't be Yena and friends, i think is gonna end okey.
I know she wasn't add to group , i meant she was add to the debuting unit