NEC SL1100 User Manual page 10

Conference Calling
To use this feature call the first user you want in the conference call, press
the Conf soft key to initiate a conference, dial the next party (with 9) or
internal number. Once they have answered you will see ADD press this
again and you can then add another party or press the BEGIN soft key.
You can have up to 16 parties (including your terminal)