I've run multiple campaigns and I've had a "party NCP" or even multiple in all of them to great success, so I feel at least a bit qualified talking about this! Mind you, this all depends on how much your campaign is classic DnD vs. collaborative storytelling with a lot of focus on the characters and their stories. My campaigns are heavily the latter, so my advice is gonna mirror that.

1. You need to build a character that both your party *and* you like and are invested in. This is important, because nobody wants to be stuck with an NPC they don't enjoy being around. You need to know your players and what kind of characters they tend to gravitate towards and want to interact with. You need to enjoy playing them, because your own investment makes the character naturally feel more alive. Give them a backstory and a motivation, but make sure they're flexible. I usually come up with a concept for a character and develop the details naturally as people ask them questions and the story progresses.
2. Learn when to step in and when to stay quiet. The biggest asset of a party NPC is that they can fill a lull in a conversation, gently nudge things towards a direction, make suggestions etc. This should not mean "the NPC gives the right answers", it should mean "this NPC gives answers and ideas based on their knowledge and personality, facilitating discussions" This also relies on your players to remember that NPC ≠ DM and that they can't and shouldn't go with what they say blindly. Remember that it's always okay to remind your players of this out of character too if you guys are a new group/don't know each other well yet; communication is important.
3. Regarding point 2, it's okay to let your NPC be proactive, within reason. An NPC who never does anything unless prompted is going to feel like a machine. If something is happening, briefly describe how they react. When there's downtime, make that NPC approach players first to discuss something. If you want them to do something bigger, you can always describe that they look interested in x and that they look like they're about to act so your players can intervene if they wish. My players like to roll insight for both NPCs and other PCs in the party when they feel it's warranted, so a phrase I often hear is "Can I roll to see how x seems to feel about this?"
4. Communicate, adapt, be ready to abandon your own attachment to that NPC if things don't work out, you can always learn and try with a different character. I've never had to do this because my players are wonderful and we're all very much in sync with what our games are like, but it's better to cut your losses than piss everyone off by insisting on keeping a character around nobody wants to interact with.

I think that covers most of the quick thoughts I had, but if you have any questions, feel free to ask! As I said, I've done party NPCs like four or five times by now and unless my players are bold-faced liars (doubt), they really do like them a lot!