The 5324 has to add all the call into the conference.
How that is done can differ a bit.
1) the 5324 hits the xfer/conf button, calls next party and hits xfer/conf again.
2) The outside party can call the 5324, it rings another button on the phone, the 5324 put the conf on hold and answers the ringing line, put the line back on hold, joins the conf again, hits the xfer/conf key again, dials the remote hold retrieve code+holding line, hits xfer/conf button again. (take a deep breath and do it all again for additional callers)
If you're going to be doing a lot of this then you really want to buy an AWC.
Ralph