One of the group (Full Members specifically) all members in that group are banned. When i check banned users there is no one in there and when i click lift ban, it says user doesn't exist. All of them have crossed mark next on their username.
How to fix this issue?
Given that you're using a third party importer to import data, it's likely not setting up all of the data correctly. (In this case, it's setting one field to say the user is banned but the other data is missing.) It would generally be best to address it at the importer level.
I'd guess you could set user.is_banned to 0 for all users if you wanted to lift the bans (assuming the xf_user_ban table is empty).