Group ID's not appearing in the "From" dropdown menu.

Group ID's not appearing in the "From" dropdown menu.

Hi
I am trying to get group e-mail IDs to appear in the "From" field's drop-down menu for those members within that group. 
I have tried making sure the field "Send as Group" is set to "Allow" for the users. 
Also in the advanced settings for the group the check-box for " Allow members of the group to send email using Group Email Address as From address."  is checked.  But the group ID's are not appearing in the "From" drop-downs.
Am I do something incredibly stupid here or have I missed something out - I need to get this sorted as soon as possible.

Thank you.