People not available on 'Select People and Groups' for Task List

I have an odd permissions error on my teamsite where we have a Task List and only 2 of us from my team are showing as being able to have tasks assigned to. If I search for anyone else, the following text is presented:

The search did not return any results from the group IT Services.

I know for a fact that we have other lists within this teamsite where these other members of our team are able to view and create content so not exactly sure they they cannot have list items assigned to them on this task list??

We have a permissions group called 'IT Services' that have Full Control permissions to the teamsite serving the task list.

Any help on why this is not working would be very helpful.

Cheers Chris