I need to send Twitter messages to groups and if I do will the tweets be private and not re-tweet-able?
As of now short answer no. A possible solution would be to create another twitter account. Protect the updates and have the people you want to communicate follow that account. As far as retweeting... I believe this is always possible. Even with protected updates.
Alternative would be to send a bunch of direct messages.
Might need to choose a different medium for communication if this is necessary. Such as email.