I seems like Safari TP 33 has no support for simulcast streaming, which would be required for multiparty scenarios. This is supported in Chrome and Firefox for VP8 and there is a patch in progress to add support also for H.264 
The standard way to add simulcast support would be using RtpSender encodings + RID header extensions in the RTP packets 
There is no way to enable simulcast in Safari for H.264.
Safari supports simulcast for H.264 (and also VP8 if support for that is eventually added)
Build Date & Hardware:
Safari TP 33 on macOS 10.12.5
FYI, there's an "Intent to Implement and Ship: Simulcast with PeerConnection AddTranceiver API in WebRTC" for this on blink-dev now:
Simulcast activation through munging is supported.
The plan is to transition to addTransceiver
Created attachment 418257 [details]
Committed r271797: <https://trac.webkit.org/changeset/271797>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 418257 [details].