Adding your friends to your facebook group is no longer a matter of manual one at a time adding though it is subject to many restrictions.
Adding up to about 900 people per day from your facebook account is about the most that you can do, you will be restricted for a few hours but that`s normal.
The fastest way to add your friends to any group that you are a member of is by using the “Group Invite All” google chrome extension that you can find in the chrome store.
Simply add the extension to your chrome browser on your laptop or desk top “not supported in mobile” and click the new extension icon on the too right of the screen.
You must be on the group page and be a group member for it to work. The process will stop at 900 which is usually safe, you will get blocked but not for long. You can use this extension every day
To increase the size of your group effectively you can also get your friends and family to help you by adding there friends to your group.
Though it is going to take 3 days to complete by doing this every day supposing that you have 2500 friends. But the benefit to your group is well worth the time spent. Plus getting help from your friends with thi sis really going to speed things up.