How to see all contacts on iphone

Can you post a screenshot?

If you are in the Groups option, there should be checkmarks next to the Groups you wish to see. No + sign is available there, as you cannot directly add Groups there.

If you see the word "Groups" on the top left and a + sign on the top right corners, then you are in the Contact view. Not Groups view. If no contacts are showing up there then you may not have any Groups selected to show.

If you tap the Groups word at the top, it should then open the Groups page. Where you can select which groups of contacts to show in the Contacts view.

Make sure at least 1 group is selected there.

https://support.apple.com/en-us/HT201398

Open the Contacts app, tap Groups in the upper left, and verify that all Groups are checked.

There should be a "Done" option at the top right corner. Tap on that to go back to the contact list, after making sure the groups you want to see are selected.