Sharing an Activity in Mercury is done from inside the Activity record. You can access Activities from the Activities menu or from the Timeline of any record.
In this example, the Activity is being accessed from the Activity menu.
Click Activities > Click the Subject to access the record of choice
Once inside the Activity Record, click Share > Email Link or Copy Link.
The Send link window will open.
Type to Search for the User or Team > Type a short message > Click Send
Clicking Copy link will open up the Link Copied window.
Click Copy > then paste that and share it as you need to, eg. Teams or a similar chat platform.
There is also another option when sharing an activity and that's Manage the Access, by default the Activity will only be accessible if the User you are sharing with has access.
If they don't have access then you can manage that by clicking Share > Manage Access > Search for the user/team that will need access > Click the name
Click to put a check next to the name > Choose the permissions you'd like to grant > Click Share