Group Membership - Placeholder

Adding the "placeholder" attribute to the multiple select for "Group Membership", in order to add support for the feature in the "Select2" library.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -