In the Summer '18, Salesforce added the functionality to mention a record within a chatter post. This is available across all of chatter but I'm going to show you an example in a chatter group. This is the sales chatter group and I'd like to mention an interesting opportunity that Ive been working on. To do so, do a forward slash and start typing the name of the record. You can then select your record, and just like with any chatter post you can type your message and hit share.