Yes, you can boulder by yourself (I find bouldering more fun anyways). Most gyms have sign up sheets to connect solo climbers with others. Even better, when you sign up at a gym you'll probably receive guest passes, take a friend some time and they'll probably want to sign up themselves because its so fun. My group has increased from 2 to 4 over the last few months.
EDIT: For those who are unaware of the climbing terms, here are some brief descriptions:
Bouldering: Climbing solo, with no gear, usually up a wall ~8-12 ft (with mats below, in case you fall). These routes are usually tougher focus more on technique/strength rather than endurance.
Top-Roping: Climbing with a partner (one climber, one belayer), both partners must have the proper gear and tie in to the ropes in order to climb. These routes are usually 20-50ft and require a good deal of endurance to reach the top.
EDIT: For those who are unaware of the climbing terms, here are some brief descriptions:
Bouldering: Climbing solo, with no gear, usually up a wall ~8-12 ft (with mats below, in case you fall). These routes are usually tougher focus more on technique/strength rather than endurance.
Top-Roping: Climbing with a partner (one climber, one belayer), both partners must have the proper gear and tie in to the ropes in order to climb. These routes are usually 20-50ft and require a good deal of endurance to reach the top.