By default, you can have up to 500 members in a group. We aren't able to support groups larger than 500.
While it's possible to have groups with 500 members, groups larger than 200 people can get noisy. GroupMe is reply-all, so whenever a member replies to the group number, everyone in the group will get a text message or push notification. Additionally, everyone in the group has to respond to the group at least once in order to stay in.
Members are encouraged to text #stay to stay in the group, which doesn't send a message to the whole group.