A friend of mine has been on SSRIs for a while and we've tripped together multiple times. However, he says he needs at least double a normal dose to trip. If I take two tabs, he takes 4 and our experiences are pretty even in intensity (so he says). I'm NOT in any way saying to start off with a big dose just based on that anecdote alone. Same for anyone, start at a comfortable dose to gauge the effects.
As for trip sitting, make sure you stay positive and engage the tripper if things start to get difficult. A friendly and reassuring voice can work wonders. Have a benzo on hand in case of shit-hitting-fan scenarios. Psychedelics make people highly impressionable, so if you have a positive vibe going they will pick up on it.