[FEATURE] Admin add member to group

[shurwit]

Is your feature request related to a problem? Please describe.
Currently to join a group the user hoping to join will need to search for, scan, or select the group and submit a request. It would often be easier for an admin to send out invitations to the users they would like to create the group with.

Describe the solution you'd like

We should build an API that allows admins to invite users to their group. To do this we would need to implement rokwire/core-building-block#575 to allow the admins to locate the users they wish to invite. They can then send the account ID of the user they would like to invite to the Groups BB. The user should then receive a notification and see the group with a new "pending" status. The user can then choose to join the group or not. Until the user joins the group they should not receive any notifications related to this group.

Edit: After further discussion, we have decided that we should just add the member to the group without invitations