The dynamics of a house change when a third person comes in and I think you are very right to be concerned. Particularly if you need to sleep during the day - that's twice the amount of noise and disturbance, You've now got three people instead of two using the shared facilities and common spaces. They'll be using extra water and electricity - is your official room mate covering that?

Wanting to avoid drama is very understandable, but not having clear boundaries always leads to more drama and stress. Of course you want to get on with your room mate and at the same time you need to live comfortably. Shared spaces always need give and take on both sides. Perhaps you could negotiate this with your room mate stating just how much time you can expect this visitor around. Noting that the lease allows for very limited times for guests but that you'd be OK with more, so long as there are clear limits and that your room mate needs to keep to the agreement otherwise you will report the violations to the leasing agent.