Grammar mistakes and typo in README.md

There are a few grammatical errors in the README.md and a typo

Fix the following lines:
- `Teams allow people` -> `Teams allows people` - noun plural, no "s" in verb
- `those groups of people ` -> `those groups` - we have already established in the line above, that this is about groups of people
- `sharing purpose` -> `sharing purposes` 
- `CireclesManager` -> `CirclesManager` - typo