To transfer the vertex group (pin) of the instance object (flower_petal) to the flower object with the Geometry Nodes modifier (Flower_GN.001) you need to do the following:
- create the pin group on the flower object (Flower_GN.001)
- add a new input slot (Pin Group) to the Group Input of the flower GN (in the node editor)
- add a Capture Attribute node between the Object Info (instance object) and the Instance on Points node
- wire the Pin Group input to the Value input slot of the Capture Attribute node
- put the Realize Instance node after the Instance on Points node. (It does not work after the Join Geometry node)
- in the Geometry Modifier settings select pin as value for the Pin Group
- select the pin group in the Cloth settings.
- check if the pin group on the flower_petal (instance object) is correct. Note that pin is not the same as Pin when you type the name.
The nodes in black frames have been changed/added.
Vertex group and modifier settings:
Please give the pin group a bit more weight otherwise all the petals hang down limply ;-)
Note about Blender 3.1 RC -
When you open the blend file in Blender 3.1 the Realize Instances node gives you a warning about "Legacy Behaviour". In the side panel (N) > Node > Properties there is a checkbox. You need to uncheck it to make the transfer of the pin vertex group work.