Create Group with initial members

[clintfred]

As as application developer
I want to be able to specify the initial group members on create
so that I don't have to make additional calls to set up the membership

AC:

• GroupCreateOpts should contain both add_as_member to mark the calling user as a member and members: Vec<UserId> to allow other members to be added
• return structure for group create should reflect the membership.

I'm uncertain if the web service supports partial failure for adding members on create. If it does, the users that failed to be added to the membership on create should also be included in the result.