Short Answer 
Cinna’s request to involve Brutus marks a critical point in the conspiracy, prompting Cassius to devise a strategy to communicate with him effectively. Cassius instructs Cinna on letter delivery methods to capture Brutus’s attention and ultimately directs a meeting at Pompey’s Theatre, underscoring their collaborative planning and urgency in persuading Brutus.
Step 1: Cinna’s Request
When Cinna approaches Cassius with the suggestion of enlisting Brutus to their cause, it becomes a pivotal moment in their plot. Cinna expresses a desire for Brutus to join them, which motivates Cassius to take action. This request triggers the following key developments:
- The need for communication with Brutus.
- Increased trust placed in Cinna by Cassius.
- Willingness to manipulate Brutus through letters.
Step 2: Cassius’s Instructions
In response to Cinna’s request, Cassius gives clear instructions on how to deliver his message to Brutus. He specifically mentions where and how the letters should be positioned to ensure Brutus’s attention. Key actions outlined include:
- Delivering a letter to Brutus’s official chair.
- Throwing a letter into Brutus’s window.
- Using wax to attach a letter to Brutus’s statue for visibility.
Step 3: Meeting at Pompey’s Theatre
After giving the delivery instructions, Cassius directs Cinna to meet him and the others at Pompey’s Porch. This is significant as it indicates their gathering place and strengthens their collaboration. The line demonstrates:
- The importance of location in their conspiracy.
- The strategic planning within their group.
- The urgency of their initiative against Brutus’s reluctance.