Just ask here on the boards if people want to do them together with you. It's not that hard. I always play Solo except when certain missions require me to play with 1 or more players. I went to the boards, and first went to see if someone is looking for someone to play with, I found one summoner and added him, so did 4 others, and completed the mission.
Riot does think about us Solo players, I base this judged on the amount of tokens you get from completing them. As far as I can remember only 2 missions require you to play with 5 men premade. And these 2 only give 5 tokens each. They might as well have given 20 tokens since it could be considered hard to complete these missions, but they didn't. So stop bashing them for ''not caring'' about Solo players. And who knows, maybe you will even find a good friend by playing with strangers.
Riot is doing it's best and put time and energy into making events for us, we get rewards for participating. Why be so ungrateful when Riot is only trying to promote team play by making a couple of missions team play based?
Just needed to get this off my chest.
Have a nice day