You have the ability to configure for each contest whether you need to be in a member group or whether anyone can enter.

To configure who can enter your contest go to the contest you would like to configure.

Select “Manage” for the specific contest and navigate to the Settings tab.

Scroll down to the security section of the contest and click into the “Which groups can enter the contest” text box.

Esnure that the "Everyone" group is selected. Once this is complete entrants will no longer need to register to enter the contest. To change so that people need to register to enter of see the contest repeat the same however this time select the “Member” group. Note all registered users are added to the “Member” group by default.
If you want to allow people who do not have access to the contest to register you must enable the "Allow new users to register for this contest" toggle switch. You can then specify which groups newly registered users are added to (additional to Member group) and then these can be added to the "Which groups can see the contest" and the "Which groups can enter the contest" setting.