When adding interactions from the constituent tile on a group record the add interaction form used for individuals appears. This form only allows for you to add the interaction to the record you are viewing. If you add an interaction from the interactions tab on a group record you get a slightly different form that allows you to choose a constituent from the group.
We're currently evaluating this issue for a fix in a future release.
Steps to Duplicate
1. Go to Constituents > Constituent Search 2. Find any Group Record 3. Select the Documentation and Interactions tab 4. Choose the interactions subtab and Click the Add button 5. The form that appears allows you to choose a constituent at the top 6. Close that window > now click the add Interaction link in the Documentation and Interactions tile 7. The form that appears only allows you to add an Interaction to the group record, there is constituent drop-down