If someone is doing a family tree and someone doesn’t come up does that mean the person isn’t in the family ?

I hope that's true but it probably isn't.

Your question is not very clear........... "someone doesn't come up" means what?

Researching your family tree means you research each and every record of each and every person so you can prove each and every person in your tree should be in the tree and if you do not then you are not 'researching' if you mean  copying and pasting from someones online tree, then that is not researching your tree, it is copying and pasting

ONE wrong person in any tree means you no longer have your ancestors, just a collection of names, dates, places that are nothing to do with you.... which is exactly what many online trees are

Blackgrumpycat's answer is correct - but I'm not sure what you mean by 'come up'.  If you've not found them through your research, how do you know their name at all? 

It depends on how far back you are.  There are plenty of mis-translations on every family tree research site.  Writing can be hard to read at times and an s might look like an f or a j like a g, so that could completely change a name.  More recent records are less likely to be wrong, but that is not always the case.

I lost some of my family for a while, but rediscovered them due to mis translations.  On sites like Ancestry, you can read some of the originals, so if the translator has made a mistake, you can check the original to see whether it is your relative or not.

With the person you may be looking for, if you think you know who their parents are, then check their records.  If you can't make a link, then it could be someone with the same surname. 

If it is a living, recent person, then it will be more difficult to ascertain if they are family or not without asking them or supposed close family.  Recent records are less likely to be available to everyone.

Are you doing this on Ancestry.com? The hints from other members are often wrong. Or there may just be no records. 

no, they just never put themselves in the family tree

No. The person doing the family tree could be a poor researcher.