Leave somebody out in the cold
Definition 1 of 2
Pro Tip 1/3Social Exclusion
Implies someone is being left out on purpose from a group or a fun social gathering. 
Even though Sarah was part of the team, they left her out in the cold during the celebration dinner.
Pro Tip 2/3Not Accident
Highlights the intentional aspect of the exclusion, not by accident. 
Mark felt left out in the cold when his classmates didn't share the project details with him on purpose.
Pro Tip 3/3Emotional Impact
Conveys the feeling of rejection or hurt from not being included. 
Being left out in the cold by her closest friends made Emma feel unwanted.