The official FAQ directs intended participating family members to be included initially. It does not provide enough detail here to promise an equivalent later-addition route.
Resolve uncertainty before submission
Bring unresolved plans to counsel with the family’s desired move dates and reasons for waiting. Ask about the actual options, costs and consequences before excluding someone. Do not treat a future intention to reunite as confirmation that the person is already covered by the principal’s case.
Deferring a decision about someone tends to convert a straightforward question into a difficult one, so resolve uncertainty before submission wherever it can be resolved. Where a family member is genuinely undecided, establish what the consequences of each choice would be: whether they could be added later, what that would involve, what it would cost and whether anything about their position might change in the meantime. Ask particularly about anyone whose eligibility depends on a threshold they are approaching, since waiting is not neutral for them.
Get the answers from current official materials rather than an adviser's general impression, and record what was said and when. A family that has understood the consequences and chosen to proceed with some members is in a very different position from one that assumed the choice would remain open.