I don't remember where I read it, but–if I'm not mistaken–CE does not allow arrays or lists as method parameters. Don't ask me why. The solution I remember seeing was to pass an object to the method with the list as one of the object properties.
Then your method would look like:
public List<GroupMembers> insertGroupMembers(MembersWrapper membersWrapper) {
for (GroupMembers groupMember : membersWrapper.members) {
mgr.persist(groupMember);
listMembers.add(groupMember);
}
…
}