Add scottrigby and CrisNevares as admins of cncf/communitygroups

@hspease99 met with myself and @CrisNevares about helping with admin support and other automation tasks for the `cncf/communitygroups` repo. This issue is for transparency.

There may be others who could also help, and it may make sense at that point to create a GitHub team (eg `@cncf/community-admin-support`), but to enable initial support the easiest step we discussed would be adding us both as collaborators on this repo. The `Admin` role would allow us to help with repo settings (see [Repository roles for organizations](https://docs.github.com/en/organizations/managing-user-access-to-your-organizations-repositories/managing-repository-roles/repository-roles-for-an-organization#repository-roles-for-organizations) for list of permissions per role).