Multiple people can sign up for events using "Group Sign Up" Self-Book template until the event reaches capacity. By default, your Group Sign-Up template limits your event signups to 100 people. However, you can change the capacity of your Group Sign-Up events by editing your custom Self-Book template. If you don't want to have a permanent change of the capacity you can change the capacity of a single Group Sign-Up event in your calendar.
First, you'll need to create an event that has your custom Group Sign-Up template. In that template, you'll need to make sure that the "Hide 'Review' link from owners/editors is unchecked. You can then create an event and add your Group Signup template to that event. Once you save your event you'll see a blue "Review" link appear in your calendar next to your Self-Book event. You can then click on this "Review" link and modify the capacity of that single event. Follow the steps in the image below:
Alternatively, if visitors have already started signing up for your group event and you want to change the capacity, you can follow these steps:
Why don't I see the "Review" link for my event?
If you create a repeating Self-Book event with the Group Signup template you'll notice that the "Review" link doesn't appear next to the Signup link. You'll need to break your repeating events into single instances by making a simple edit (like adding a space or comma to your event text) to one of the events in the series. Once you have separated an event out from the series a "Review" link will appear.
See the image below: