If you’re failing to add a user into a SharePoint group, either by going into the group and then clicking on the ‘Add User’ button, or by trying to grant access to a user by adding them into the group from the ‘User Permissions’ screen, you may want to take a look to the SharePoint group’s settings.
The reason is that even if you’re granted ‘Full Access’ to the site collection, the only people who can add users into SharePoint groups are the SharePoint group’s owner, or members of the SharePoint group that has been defined as the owner of the SharePoint group you want to add users to.
In the SharePoint group that you want to add a user, access the group settings and check who has been defined as the group’s owner. Unless you want to be the only person who can maintain the group, what you’ll want to do is define a SharePoint group as the owner of the specific SharePoint group.
Typically, when you first create a ‘<Site Collection> Owners’ group, you add however many users in the group and then change the group owner from yourself to the ‘<Site Collection> Owners’ group itself.
That means that all users in the ‘<Site Collection> Owners’ group will be able to maintain the group.
When you then create a ‘<Site Collection> Members’ group, you’ll add however many users you want to the group, but you’ll probably have to change the owner of the group to be the ‘<Site Collection> Owners’ group again. There’s no logic in allowing Members to maintain a SharePoint group.
