We have a constituent that we know is in a group but does not show up in the group list. We can search for her, though, and her name appears, and she's in the group on her Constituent360 profile.
The list that appears under the Members tab when viewing a Group has a display limit of 500. Regardless of the number of members in the group, only 500 constituents will appear in that list. This does not mean the constituent is not a part of the group or will not be treated like other members of that group. This is only an upper-limit on the number of group members that will appear in the list.
You can confirm membership by using the search bar on the Members tab. Even if the constituent does not appear in the list when you scroll through it, they will show up in a search. Additionally, you can check their Constituent360 profile to see if the group appears in their Groups tab.