Minecraft FTB assign more Ram?

I play with some friends with the MC Infinity Evolved mod pack i have to start it via twitch who knows how i can add more ram since it lags a lot.

I have a Mac MacBook Pro (Retina, 13, mid-2014
Processor: 2.6 GHz Intel Core i5
Memory 8 GB 1600 Mhz DDR3
Boot volume: Macintosh HD
Graphics card: Intel iris 1536MB

Do you also want to stream and ask because of the RAM google xD

Ne does not want to stream but if you start it you have to start it via twitch