As seen on the Arsmagna page for example:
For anyone adding information to a group page about each member of the group, You can find the code for the show/collapse function on This Wiki Page
It is under the subheadding "Selecting collapsible content " towards the end and has red text in it.
You do not need to include the "This text should be visible as well. " part of the code, which should be at the end of it