Will all clients have the same Private Key?
Ideally, no. Each client should have its own certificate, and each certificate should have a unique private key. If I remember correctly, standard OpenVPN configuration even enforces a maximum of 1 connection per client name (although this can be disabled).
So although a single certificate shared by multiple clients is technically possible, it's very much not recommended.
Thus the same encryption?
These certificates are not used for encryption – they're used for authentication. The per-session encryption key is generated independently from that.
Is it safe to share the .ovpn file with everyone that should have access to my VPN?
That's practically the same question as "Is it safe to share the username and password?" And usually, no, you wouldn't let multiple different clients use the same username, because then it becomes very difficult to protect the credentials from leaking where they shouldn't and revoke them when that happens.
There are exceptions to this, e.g. multiple devices owned by the same client could use the same account (whether it's by username or by certificate), but as a general rule, primary credentials should never be shared between users.
(Secondary credentials, such as OpenVPN's HMAC "tls-auth" static key, serve a different purpose and sharing them between users is usually okay and sometimes unavoidable.)