It is likely that these members have no credits available to transfer at the time you are attempting to transfer.
The amounts you see listed next to their names (before you attempt to perform the Group transfer) are credits accrued in the account, not necessarily those that are available to transfer now.
If you click on the 'History' link next to the member's nickname, it takes you to their respective credit histories. Here you should see the credits this member has earned, some may not be transferable now but will be in future, at a specified date (shown on the page).