Working out solo can be more convenient, peaceful, and a great time for self-reflection but those reasons won't do anything to increase your calorie burn. If weight loss and toning up is your main goal, here are some reasons you might want to find a group to sweat along with.
Variety: When we exercise alone, we tend to get in a workout rut, doing the same exact biking loop or toning the same muscles in the same old strength-training routine. When you work out with other people, they open your eyes to exciting ways to sweat it out like new routes you can hike, or more effective ways to tone your glutes. Using your muscles in different ways challenges them more, offering you a better burn while also preventing the dreaded workout plateau.
Friendly competition: If you've ever gotten a taste of rivalry, either from a team sport or seeing if you can do more push-ups than some stranger you see at the gym, you know it's a powerful fuel that can make you work more intensely than you could alone. You can compete openly with a buddy, like seeing who can sprint the fastest to the telephone pole, or you can pick a random person in your Body Pump class and see if you can do more squats than them. Pushing yourself gets your heart pumping faster, your muscles working harder, and in turn burns more calories.
Longer workouts: We've all been on a solo run with the incessant thought, "I just want to be done!" When you're on your own, it's easy to hop off the treadmill whenever you want, but when you're chugging along with other people, throwing in the towel doesn't seem like an option (you wouldn't want to give up in front of everyone!). You're not only more likely to do a full workout when exercising with others, but if they're really serious, you're bound to work out even longer than you would on your own, burning more calories.
Excitement about working out: Solo workouts can get lonely (even with Macklemore pumping in your ear), making you bored and not as energetic to give it your all (or to give it at all). Finding others to sweat with can breathe new life into your workouts because nothing beats the power of working out in a group. You feed off everyone's strength and energy, which inspires you to push harder, and you have fun doing it. And you remember that feeling, which inspires you to slip on that sports bra again and again. You'll end up burning more calories per workout, and if you're motivated to work out more often, you'll in turn burn more calories per week.