Does contributor level going higher when i do Group only giveaways?

Yes, if more then 5 entries

https://www.steamgifts.com/about/faq

If a giveaway is invite only, region restricted, or limited to a whitelist or Steam group, it will only add contributor value if it receives 5 or more entries.

You won't get MORE CV for group only giveaways. CV is calculated according to the price and boundled status, but in private/group GAs you need at least 5 entries to get it.

I think the correct answer is no. Compared to a public (for everyone) giveaway, a Group Only giveaway does not give higher CV, but just the normal CV (if more than 5 entries and no CV if less than 5 entries). Read the the FAQ (https://www.steamgifts.com/about/faq), there's a section called "Contributor Value and Levels" that gives all details about CV and giveaways.