I do not believe this is possible in the current release. Assuming multi-line phones, you could install additional extensions on same and make them a part of the group or agents. Personally, it would drive me nuts being in a call and then having another call coming in (to my extension) and especially so if other agents/members are available to handle it. I guess a way around this is to set-up a second group/Q containing only the added extensions. It may also make the ring strategy problematic for those using the call center editions (PRO).
The groups/queues are generally designed to be manned by enough people such that the need for a person to handle more than one call is not needed. If the demand is greater than the supply, then use of a queue may be more appropriate as it will hold the caller in "suspense" pending an agent becoming available whereas the group will move the call onto the next call handling rule.
Granted, there may be some times when no matter how well planned, the call volume may exceed the capability, but I guess it depends on one's willingness to put an existing call on hold in order to pick-up the second call and then decide to either continue with the second call or put the second call on hold and go back to the first caller.