Because group follow it is a specific way of handling machine names (they contain dashes) , it is impossible to target a specific form id with a hook and quite often we have to use a general form_alter hook, which is called for every form and this is a bad practice due to performance.
Try to hook any group content form, in our case for exmaple we get something like this "group_content_group_type-group_membership_delete_form"
My suggestion to override getFormId for GroupContent entity class and replace all dashes to make it this Form Id suitable for php functions.
Needs review
1.0
Code
I think, it would also nice to have base classes for Group content form and Group content form confirmation. This way we are sure that form id is correctly handled. Submodules can use it.