Salesforce.com give 5,000 chatter free licenses, it is good for organisation to use Chatter for collaboration as it is free. But, adding hundreds or thousands of user as member into many Chatter group is not fun, if do it manually.
If you are Salesforce admin, here tip to make your life easier :)
You can use Data Loader to mass add member to Chatter group.
1. Open Data Loader and select Insert
2. Select "Show all Salesforce objects"
3. Select "Chatter Group Member"
4. Map "CollaborationGroupId" to Chatter Group Id and MemberId to User Id