Old Telephone System
On the Siemens phone system you can use simple conferencing at no additional cost other than normal outgoing call costs.
To start conferencing initiate a call as normal. Place the caller on hold using the Recall button. At this stage you'll get a high pitched or broken dial tone, that's fine just dial the next number not forgetting the '9' if it's external. Once connected, press recall again and enter *4, this will join your three parties together.
Repeat the Recall/*4 process again until you have your 5 participants.
New Telephone System
On the Cisco phones the process is much simpler, dial your first number, press the conference button (it looks like s mall group of people) and dial your next call, repeat that process until you have your 5 participants.