Groups and Members database: Which of these two options is a good design? Each group can have either 1 or 2 members, no more, no less. Every select of group data will need to retrieve the member data. The eventual state of each group will tend towards 2 members (full group).

Group
group_id | ...

Member
group_id | user_id


vs

Group
group_id | user_id_1 | user_id_2 | ...
The first one is probably the best option.

Members of a group is a kind of data that could increase some day and if you keep the first option you will only need to add a new row in the Members table.
Also you can remove or add users from a group without messing with any possible Groups indexes and without needing to update which is a more expensive action.