I know both ways: keeping it hidden and telling everyone.
With DD#1 March 2012, the day I found out, after seeing a text early that morning, with my WH texting and me a steady stream of denials, justifications, and pleas, I went to my parents after work. I was in such deep shock and bombarded with frantic messages from WH, that I seriously didn't trust myself to read the words correctly. I showed them everything he'd sent to me, told them about picking up his phone that morning, and asked, "What is this?" But I quickly covered it back up and minimized how bad it was, that it was just poor boundaries versus the huge emotional affair(s) it was.
Over the next few months, my slow motion breakdown made people suspicious, but they never would have pinned it on WH - everyone adored him. It almost killed me to carry on the duplicitousness of maintaining "who he was" to everyone outside me versus what was going on between us - rug sweeping, false R, escalating fights, etc. Our older children knew something was up, but it wasn't until the end of that year that our two oldest teen daughters heard enough to clue in.
When DD#2 hit this March, I literally walked to my parents' house with WH following and pleading with me not to tell within 10 minutes of seeing the chats in his email. I said I wasn't going to protect him anymore and didn't, kicking him out that night.
Everyone found out, including our older teen children. It was easier to be open in someways, harder in others: I got the range of responses from sympathy to denial to frustration with me that I was casting his behavior in an unnecessarily bad light.
Now that we're in R and he's back in the house, some people behave how they did prior to knowing, some don't.
What's most important is how you feel. For me, I had to have it out in the open this time. I ended up with stress related health issues from keeping it all hidden and putting on a good face last year.
There isn't a right or wrong answer. You know your situation. You know your marriage. Do what makes YOU happiest.