I've been learning to play the acoustic guitar for about 5 months now and I was thinking of joining this guitar group that does guitar picnics every weekend in the park. They welcome beginners, so I thought it would be cool to hang out with other guitarists and share my interest with them. That way, I can meet other guitarists, make friends and at the same time, learn more about the instrument from these people. But I'm kind of concerned that 50 degree weather might be harmful for my guitar. I heard that cool temperature could be bad for the instrument, but maybe I'm just being paranoid. It's just that I hear too many "care for your guitar tips" that I feel that the acoustic guitar is the most sensitive instrument in the world. So, it gets me kind of nervous.

So, is 50 degree weather harmful for my acoustic guitar, especially when it's exposed to that temperature for several hours?
That depends a great deal on what type of guitar it is. I wouldn't attempt it with a high end all solid wood instrument. (Unless I was getting fairly paid). But, with an all laminate guitar, the biggest danger is your fingers stiffening up from the cold.

50 degrees isn't terribly cold. With that said, there is also a great deal of hysterics regarding providing a museum environment, (72 degrees, 42 % RH, day in, day out, all year round), for better acoustics.

Although, I do think trying to keep the guitar in tune could be somewhat of an issue.

Bottom line, campfires require beaters, leave the Martin at home.
I think it will be fine. I often play outdoors at those sorts of temperatures and have had no problems.
I would imagine your guitar will be fine, but I would never play in that cold of temperature, because it is too cold for my hands. Maybe if the sun was out and beating on me and there was little to no wind it would be ok, but sounds too cold for me.