Group ideation works most the time with all people at the same place at the same time. But you can also do ideation without being at the same place, or without even creating ideas at the same time.

Asynchronous ideation

The idea box that many companies had back in the days in their offices is what would be called today by academics an asynchronous ideation tool. Basically it's a tool that people can use at different times. 
When I know I don't have much time for an ideation workshop, I often ask participants to already create a list of ideas (with the help of a few guidelines) before the session. This is another example of asynchronous ideation where every team member creates ideas at his favorite time.

Remote ideation

When it's hard to meet all at the same place, because you live too far away, remote ideation can be something really interesting too. Basically, instead of meeting live in person, you meet via Skype or another specialized tool like Miro.