This could get dangerous but I'll bite
Syncing is normally done during early hours of the morning. YSE will connect to your 3300's and read all the data into it's database.
If your media server (3300) on YSE is set to read/write then changes will be written to the 3300, so you can create users and groups to a certain extent.
If it is set to read only then you would have to perform a sync to get the data into YSE (either manually or waiting over night)
Creating things on YSE won't write to the 3300 if in read only.
What are you trying to achieve. i.e. A Call Centre Queue with ACD agents, ring groups.
YSE is really the reporting package. If you only want to create a simple group then you are probably better off creating directly in the 3300.