Short answer
In Settings > Objects > Deals > Setup, Split Deals among users lets you share credit. Each user's percentage of the amount shows on the record and in forecast views. Users who worked on a deal but need no credit can be added as collaborators instead, which is a separate setting.
1. The setting
Both options are off in my demo portal.
2. Before turning it on
- Agree split rules with sales leadership first.
- Forecast totals use split amounts per user.