When you add a sub grid on any entities form and in properties of sub grid, select entity as Connections (Connected from) in CRM 2013, the + symbol to add a new record, doesn't appears on the form.

Steps to reproduce the issue in CRM 2013:
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1. Navigate to Settings >> Customizations >> Customize the system >> Entities >> Account >> Form and Open information (main) form
2. In ribbon, click on Insert >> Sub grid
3. Double click on the sub grid added and filled all required details
- Name : Test
- Check the box for Display label on the form
- Under Data Source Select options
- Records: Only Related Records
- Entity: Connections (Connected form)
- Default View: Choose any options, I have selected Active Connections and selected All Sales Team Members next time while reproducing the issue.
- Set
- Save and Publish
- Reload the form
4. Now, when you open Account record, we do not see + sign to add record as compared to other sub grids.