Cap restrictions can be added to each individual also, and this allows the owner to have more control. To do this, create a mapping that stores the address as the key and cap as the value. Creating a mapping for contributions is also required:

mapping(address => uint256) public contributions; 
mapping(address => uint256) public caps;

Verify the individual cap for every token purchase to restrict users:

require(contributions[_beneficiary].add(_weiAmount) <= caps[_beneficiary]);

Create read-only methods for reading both the contributions and caps of an individual user:

function getUserCap(address _beneficiary) public 
    view returns (uint256) { 
    return caps[_beneficiary]; 
}

function getUserContribution(address _beneficiary) public 
    view returns (uint256) { 
    return contributions[_beneficiary]; 
}

Create setter functions for creating user restrictions. You can also create a batch function, which accepts multiple addresses and caps for easier assignment:

function setUserCap(address _beneficiary, uint256 _cap) 
    external onlyOwner { 
    caps[_beneficiary] = _cap; 
} 

function setGroupCap( address[] _beneficiaries, uint256 _cap ) 
    external onlyOwner { 
    for (uint256 i = 0; i < _beneficiaries.length; i++) { 
        caps[_beneficiaries[i]] = _cap; 
    } 
}