You can only apply for family reunion if you have been granted refugee status or humanitarian protection. If you have been given another form of leave such as discretionary leave, even after having claimed asylum, then you are not entitled to family reunion, but must sponsor family members in accordance with appendix FM.
If you are a refugee or have been granted humanitarian protection, then your immediate pre flight family members will qualify. This includes your spouse or partner, and children under 18 who was part of your family unit before you fled.
There is no visa fee for making such an application. The application is free of charge. It is very likely that you will have to make the application on line as most countries operate an on line application process only.
There are provisions in appendix FM for family reunion of other family members or post flight family members, but the requirements you need to meet are more difficult and such an application is not free of charge.
If you require assistance or help with any of these applications, then please contact us and we can assist with all the preparation and making the application from here in the UK.