addMembership(username, options, cb) {
      log(`Adding user ${username} to Team ${this.__teamId}`);
      return this._request('PUT', `/teams/${this.__teamId}/memberships/${username}`, options, cb);
   }
Add a member to the Team
@see https://developer.github.com/v3/orgs/teams/#add-team-membership
@param {string} username - can be one of: `all`, `maintainer`, or `member`
@param {object} options - Parameters for adding a team member
@param {string} [options.role=member] - The role that this user should have in the team. Can be one
of: `member`, or `maintainer`
@param {Requestable.callback} [cb] - will receive the membership status of added user
@return {Promise} - the promise for the http request