Is there a administrator key that can decrypt all ciphertexts encrypted with different keys by different persons in a group?
Another solution (cf to @Raoul722 comment) is the following:

It is easily feasible with a mix of symmetric and asymmetric encryption.

Assume each user $k$ have a key pair: $(K_{pub}^k,K_{priv}^k)$.
The admin also have his key pair: $(K_{pub}^{admin},K_{priv}^{admin})$ where $K_{pub}^{admin}$ is known by all.

Encryption scheme for a user $k$:


generate a random key $K_{sym}$.
Encrypt your file with a symmetric encryption device such as AES with $K_{sym}$.
Encrypt $K_{sym}$ with assymmetric encryption (RSA, ECC...) with the
$K_{pub}^k$, let's call it $K_k$.
Encrypt $K_{sym}$ with assymmetric encryption (RSA, ECC...) with the
$K_{pub}^{admin}$ let's call it $K_{admin}$.


Decryption scheme for a user $k$:


Decrypt $K_k$ with his private key ($K_{priv}^k$), he will get $K_{sym}$.
Decrypt the file with $K_{sym}$.


Decryption scheme for a user $admin$:


Decrypt $K_{admin}$ with his private key ($K_{priv}^{admin}$), he will get $K_{sym}$.
Decrypt the file with $K_{sym}$.


The sole problem is where do you store the encrypted keys: $K_{k}$ and $K_{admin}$ (maybe in the meta-data of the encrypted files? :) )

About the key privacy: In this scheme, having a asymmetric scheme prevent the admin to know each user private key.

Note: this kind of scheme is useful in company in the following scenario. Assume that everybody encrypt his files. One happens to have a lethal accident (R.I.P.), how do you retrieve his work ?
The problem that will arise is the safe storage of the master key (usually off-line and sealed to prevent malicious usages).