You need to be on a call to use this feature.  Once on a call:

1. Dial *1, then the conference room number (any number between 3000-3019)
2. Hang up
3. Local extensions can join the same room if needed or more outsides callers can be transferred to the same conference room.

Note:  A blind transfer (*1) is used when you want to transfer a call to digits with more than 3 digits. Using Attended Transfer (*2) will not work because *2 is limited to 3 digit and conference rooms are 4 digits.