To work out a formula to not see people twice at a meeting

I have an event where at the moment 30 people meet in groups of 5 and change rooms 6 times.
I need a formula that 30 people (this can increase if needed) can meet in groups of preferably 5 (although this can be between (4-6) can meet and never see each other times, happy for the number of rooms / meetings increase from 6 if needed.
Can anyone help to stop people meeting twice and only see each other once?
The aim is to meet as many of the people as possible.