For most events you can reallocate tickets up until midnight on the day before the event, or midnight on Thursday for events taking place between the following Friday and Sunday. If an event is marked 'No reallocation' this means the reallocation facility is not available and you are not able to transfer your tickets into another person's name.

This will be either because the reallocation window has closed (it is too close to the date of the event), or because the event organiser, the venue, or sometimes the artist themselves, have specifically requested the reallocation function be turned off.  

In these instances, the named tickets holder must attend the event themselves together with all other members of the group.