Feature: Add Friends to Fitness Activities

[Sahil-Mehta881]

Feature Overview
A form for people to add friends to participate in a fitness activity by inputting their @Signs. After a friend is added, the activity should appear on their home screen (see home screen feature: #2).

Why This Feature
This is necessary for the people to share health data on the app (i.e. currently in the form of fitness activity information) and promotes referral of new consumers onto Priv@teFit

Additional context
The design of this form should include a field to input a friend's @sign, submission button, and confirmation dialog. Feel free to share a design of this screen (e.g. Figma / Adobe XD) as a comment to this issue.