2 answers
Hi Justin,
If you want to incorporate both User and Contact as part of the folloup notification, create a task.
Otherwise create a chatter post, but you can only @mention users there, not contacts.
You can leverage Process Builder to do both of these.
This link will guide you through how to create a Process for posting to Chatter.
https://rakeshistom.wordpress.com/2015/03/24/getting-started-with-process-builder-part-13-post-to-chatter/
Let me know if you need any further help on this.