Depending on the purpose of your group, you can make it open, closed, or secret.
Open Groups
The content of posts in open groups are publicly available.
* Only pixiv Premium members can make open groups.
These groups are visible in search results.
Anyone can join.
Comments and other posted content are public as well.
Closed Groups
The content of posts in closed groups are only available to group members.
* Only pixiv Premium members can make open groups.
These groups are visible in search results.
Anyone can join.
However, comments and other posted content are only available to group members.
Secret Groups
Members and the content of posts are all private.
These groups aren't visible in search results.
Joining is invitation-only.
Comments and other posted content are only available to group members.