People can get added to a collection by clicking on the three yellow dots right of the collection name. There you can then select “Share Collection”. On the appearing site, you can add users by supplying their email addresses. They will then get informed via email that they got access. For each person, you can then also set what rights they have.
Admin -> Can add and remove users. Could also remove you from the collection
Tag -> Create/remove tags
Entries -> Add/remove entries and apply tags